Direct radiators can be around 1 to 3% distortion for " Hi Fi" so depends what you consider " distortion"
And pedal distortion can be, say a tube screamer full tilt around 15 to 22 % distortion.
Speaker can move so far forward and back before it reaches 10% distortion often rated as " Xmax " or linear travel
lots of standards. But usually 70% of speaker BL. So how far forward / back a speaker can move before reaching 10%
is considered Xmax. Basically the magnet only has so much control to move it a certain distance before distortion climbs
knowing fundamentals of open strings good place to start, lets just look at lower frequency wound strings. Of course depending if you use a wound G or not
with string gauge. Im rounding to non decimal for simplicity
Low E 82 Hz
A 110 Hz
D 146 Hz
G 196 Hz
Lets look at a typical underhung 150 watt guitar speaker in a sealed box, which is super fair to it not open back so it actually has more cone control.
Xmax rating as discussed before is .8mm some go to 1mm. Once cone travel reaches .8mm you are at 10% distortion or Xmax or linear travel.
Look at the red line straight across measuring .8mm =10% distortion cone movement above that is far far more than 10%
Yellow line = 15 watts already exceeded xmax by almost twice
Green line = 30 watts
Red line = 60 watts
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Your other question is below , amplifier bandwidth or AC analysis of very simple amplifier and bandwidth is close enough to 5 Hz to almost 1 Megahertz
is far far far wider than the guitar pickups, strings or speaker
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Bass is no problem for a amp. Again Voltage depending how many volts your putting into a 4 ohm load the current goes up.
Transistor can only do so much current and gain drops. So depending how many volts you wanna put into 16 , 8 , 4 or 2 ohms
That requires so much current. So again the transformer needs to be big enough to provide that current and you need enough output
transistors to also pass that current and not overheat using a heatsink. So flappy weak amps use penny pincher small transformers and
could use more output devices, instead of hammering a single pair of transistors. often done with many amps. Pushing parts count low
and using small cheap transformers. So driving difficult loads like 4 ohms using more transistors and bigger transformers for more current
will keep bass healthy. Then of course a speaker or more like multiple speakers sealed back for bass response
for serious drop D even C I wouldn't use the typical underhung speakers, something with real bass and excursion