I know some people like the way the JC120 sounds, but IMHO.
I own several classic, vintage all-tube guitar amps. IMHO most of them can be dialed in to sound pretty decent at a reasonable volume, especially if you have a decent compressor pedal. BUT! A 100W Marshall played at bedroom level does not sound like a 100W Marshall that is cranked to the max.
You could reasonably argue that a guitarist should use a lower powered amp that can be dialed in to sound and feel great at a reasonable volume. However, part of the reason why people like really loud sound is because it stimulates their endocrine system and puts them into a state of excitement. It's actually sort of an addiction, where you crank up to get a hit.
Other factors.
-A lot of guitarists have damaged their ears, which means the have to turn up even more to hear the desired tone and hit.
-As @Gearhead17 mentioned, many guitar players can't stand the sound that comes out the front of their rig. This is partly because of the way they adjust the amp IMHO, but also it relates to the type of experience they want. In essence they are looking for more of a diffuse sound that seems to completely surround them. So rather than hearing the direct sound, they want to crank the amp so loud that they can key of the indirect sound bouncing around the room....(IMHO this is the biggest part of the problem).
So, the primary problem is the guitarist pumping the room full of reverberant energy, and result is the SPL is pretty much the same at all points within a significant distance of the guitar amp. In other the SPL of the guitar is pretty much the same for all band members.
In my experience most people want to hear themself about 6-10dB louder than any other sound source. The SPL of a point source drops by 6dB for each doubling of distance. So if you are 4' from your bass rig, the guitarist needs to be 8' for your sound to drop off by 6dB from the SPL you hear. The problem is the guitar signal does not drop off by 6dB for each doubling of distance if the guitarist is pumping the room full of reverberant energy rather keying off the the direct sound coming out the front of his/her amp.
Even worse, some guitarists will try to aim their cranked amp across the stage, so you may be hit with a very bright and harsh tone that is significantly louder than the reflected sound the guitarist is keying off.
My take is if you are getting paid, you should be concerned about what the audience wants. If the audience wants massive volume then feel free to rock out. But in my experience many bands would get more work if they learned how to deliver a good show at a much lower volume. It's a different set of priorities and also a different type of experience. Those who insist the sound always has to be extremely loud are usually focusing on what they want, rather than what the audience wants. Not a very professional attitude if you ask me.
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