The jsx is great for practically any sound you could ever want.
It has more than enough gain.
Its basically down to what sound your after mind but simply swapping pre amp and power amp tubes for something closer to what you want can usually resolve any OMG I GOT THE WRONG AMP issues
The EQ section on the jsx is just amazing mixed with the resonance and presance controls you literally can get any sound you want from this amp
If it doesnt give everything you want an extra EQ stomp box will resolve any issues
I run mine with JJ 6L6GC power amp tubes and JJ 3x ECC803 1 for clean 1 for the ultra channel and 1 to power them.
The ECC803 give me that real tight bass sound for faster articlate sounding riffs and they have silly amounts of gain
I use Electro Harmonic 12AX7 for the crunch channel this gives me a deep looser sound great for vintage sounds and i get a decent dimebag type sound from it aswell
from my experience you can get basically any sound you want from any amp with just a mixture or the right valves and EQ + guitar the only difference is some amps give certain things easier
ie if you want a mesa type sound getting a mesa is the easiest way to get that sound but you can still get very close to it with other amps
This is why i love the jsx it has all the sounds within it with just a few presets and the switch of a channel i can get everything from Jazz/Blues cleans all the way upto Thrash/Speed metal/Death metal not to mention classic sounds like the Brown Sound or Angus Young tones
i just read this back and i sound like a right fan boy haha :P
anyway i hope this ramble comes close to helping anybody trying to decide on a JSX it really is the dogs danglies
just to add i play a 7 string through it and yeah its great