In 90% of instances 'the talk' is about vacuum-tube biasing for musician's amplifiers; much of this is due to biasing effect on tube life,  but other things are also affected, such as the power output and the tonal quality.  For transistorized output stages, bias is set once, and then not again ....unless repairs (including new tubes on tube amplifiers) are done.  Transistor output stages have the bias current set for a value that is high enough to eliminate distortion at very low volume levels, and low enough so that the output stage does not get excessively warm with no signal input; AND, to a specified voltage or current in some area of the circuit, that is often adjusted on an oscilloscope, using a sine-wave tone from an oscillator.  The adjustment is often made to the point that noticeable distortion disappears.  Many transistor output stages have an adjustment for a specified current flow, which may be measured in several different ways, including the voltage drop across known resistor values (1 ohm or less) in the circuit.  Transistor output stages may have other adjustments, for such as DC output minimalization, balance, etc. 

 Vacuum tube output stages may have balance adjustments besides bias adjustments.

For vacuum tube output stages, the situation can be simple ...or, slightly more complicated.  One type of design circuit uses "cathode biasing" that usually has no adjustments for bias.  That type of circuit is used almost exclusively for lower wattage amplifiers, with some notable exceptions.  For high output vacuum tube output stage amplifiers for musicians, where power output of the tube(s) is of importance (to get the most power, within reasonable tube life, and more or less a certain tonal quality) the type of biasing usually used is called fixed bias, and the needed negative voltage is supplied by a negative power supply output.  In these amplifiers, bias on the output tubes is nearly always adjustable and, as noted, there is no cathode resistor (or, the one used is very low in ohms, such as 1 ohm and exists to allow voltage measurements that, via Ohm's Law, one mathematically converts to current flow; or, for connection to an oscilloscope, or, other reason).  Some of these amplifiers have balance adjustments, and some old amplifiers from the sixties (such as from Fender Musical Instruments Corp.) are designed to incorporate a "sort of" balance and minimum hum...and, MAYBE bias adjustment ...all via just one control.   Depending on the amplifier design, there may be connection places for a meter, which may be on the chassis available to the owner/user ....or, inside, and several types of adjustments. Some have lamps to indicate proper adjustments.   Adjustments allow for bias amount, balance between the tubes, etc.   Standard settings are usually specified by the manufacturer, usually an oscilloscope and a sine wave oscillator is needed and one or two of the adjustments are made at or quite near-to, maximum clean signal output....this means that the output is watched on the oscilloscope, while adjustments are made.  A special resistor load is used instead of a speaker.   Some amplifiers are not stable, with certain loads.  This is seen now and then, and the instability is typically at a frequency much higher than the signal used for the testing.  A truly good job is completed by careful analysis of load, power, frequency, and such as burst signal or other pulse signal.

Adjustable power stages may have bias set at certain published values, often this is actually a suggestion only, and applies only to brand new vacuum tubes that, themselves, are tested on a special tube tester; for specified characteristics.   When the tube(s) are installed into the amplifier, the adjustment is usually BEST made for a minimum amount of current flow that still minimizes distortion, especially second harmonic type.   Note the difference, in that transistor power stage bias is usually set at very low signal (if any) output, while tubes bias is usually set for just before any more volume creates an overload while looking at the waveform on an oscilloscope (I do transistor stage testing at both signal situations, all on a 'scope, but I mostly do it as a check on the published settings).

Re-saying some things:
When talk is about 'higher bias', that is almost always for vacuum tubes and usually means LOWER negative bias VOLTAGE than normal, and it can result in a lowering of tube life, due to the HIGHER continuous current flow, with only a modest amount of extra power output being available from the higher bias.  The amplifier manufacturer selects bias for a certain amount which relates to a certain current flow; or, more often, selects the bias so that with whatever particular tubes are installed, the tubes are operated under conditions that reduce output distortion, yet provide a good compromise for maximum power output.  USUALLY, the important biasing for vacuum tubes is set with near maximum power output, so that at just barely into overload conditions, the sound is a type of 'clean' tube amplifier overload, and not a raspy lousy sound.   Bias that is set higher (lower voltage) is sometimes used by musicians who want a certain type of sound at and through the loudest overload (maximum power/signal output) condition.  These folks will pay the price for this, in replacement of tubes and sometimes other things.  Sometimes the biasing checks are done at both very high output & very low output.  I do this.  There are some types of vacuum tube output amplifiers that are specifically biased for extreme LIFE, those are typically for public address use in factories, offices, etc.   Quite often these amplifiers run lower B+ voltages together with cathode bias.  This can give extreme tube life, and have no drawbacks for Public Address purposes.

Amplifiers for musicians are OFTEN designed so that the amplifier has one or more 'clean' pre-amplifier channels, and also one or more 'dirty' or purposely distorted channels that have the capability via front panel adjustments for such distortion/type/etc, etc.   There almost always is both a master volume control and channel amplifier volume controls, and proper use will produce clean sound somewhat into the OUTPUT STAGE OVERLOAD POINT ...OR, the master volume can be turned down some, and the pre-amplifier gain control turned up, which will cause, on purpose, a hard overload sound (more or less).  Some musicians amplifiers have all sorts of combinations of these sorts of things, even multiple IN SERIES pre-amplifiers, which enable, on purpose, high distortion and headroom, at even low volumes.  Some amplifiers are purposely designed to have extreme control over the amount of amplification/gain. Some later large Marshall amps are like all of these things.  Marshall is NOT the only one that has produced such amplifiers...Ignater did, and others too.

Some amplifiers have switches that change bias and/or something else, for different types of sound.   There may be switches for maximum power output adjustment, or, for Triode Connection sound, etc. Some amplifiers have front panel controls for a dozen or so different type of main amplifier sound/tone.   There is a UK/England "British" sound, and many others, some called by the particular type of circuit design, or even by vacuum tube number.   Example:  EL84 or EL34 or 6L6 sound....etc.   Some settings have other descriptive names.  Some available switch settings may modify the bias, or a list of 'other things'.  Transistor output stage amplifiers have other methods, in general, for producing tonal change.  Some amplifiers have switches for power output at the overload point, and some also have switches to change the output circuit from 'beam power tube types' to triode type.

Many, not all, musician's amplifiers (especially ones over 10 watts or so) have both a power switch and a standby switch.  The standby switch turns off the high voltage to the output tubes.  It's really a good idea to have both of these switches.  I can usually add such a switch if you do not have one.  The cost is typically paid for by lengthened output tube life, fewer blown fuses and other parts, etc.   These standby switches extend your output tube's life for two reasons:

While this description/advice applies to most vacuum tubes, it is more important for the larger audio power output tubes for technical reasons.  At the very center of most power-type vacuum tubes is a central tube, usually round, but sometimes round and flattened in shape.  The central tube contains a spiral (or can be up and down thread-like) tungsten thread (or ribbon) that is heated to glowing reddish-orangish-hot, which heats the surrounding central tube to approximately the same color.  Sort of similar, if smaller dimensioned, as the toasting element in your kitchen's bread-toaster, which is usually a similar Tungsten thread, but there is no 'tiny tube' covering the heating element in your toaster.

The outside of the vacuum tube central tube is coated with a substance that likes to emit electrons when reddish-orangish hot, if proper voltages are applied to other items in the vacuum tube.  The electrons can be thought of as flowing towards the big grayish surrounding metal plate structure.  

If you turn ON the power and the standby (high voltage) switch at the same time, or the standby switch is turned on much too soon, or, there is no such switch, then the high voltage appears fast, in some cases instantaneously (with exceptions for slower-to-heat-up vacuum tube rectifier tubes which don't need a standby switch much).  The positive gray-colored 'plate' that surrounds the central tubing (at some distance) will 'draw' (attract, greatly) concentrated areas of electrons from anyplace it can.  With most power output tubes, this happens also from a certain wire-mesh-grid located inside the surrounding gray plate. While a large current flow can damage fine-wire structures such as that wire-grid, the discussion here is for the electrons-emitting central tube.  If you do not have a standby switch, then the following effect happens at every turn-on, particularly with solid-state rectifiers type of power supply, but also tends to happen with rectifier tubes that heat up fast: