Great review, lots of terrific info on the JSX. Thanks for sharing all this Darin!
I've owned a JSX head for about 18 months so I'll chime in with my experience. I love the fact that I continually re-discover how versatile it really is. I read an interview with James Brown (the guy who engineered the JSX with Joe, not the famous 'feel good' front man) in a guitar magazine and he said Joe runs everything at 12 o'clock, but cranks the bass on the crunch and ultra channels up to 8 or so. When I first got the amp I ran it that way (except turned the "Crunch" gain down to 2 or 3) and used the "High Gain" input for all my live stuff. Of course it sounded great so I figured "if it ain't broke, don't fix it!"
With my previous rig I used rack based effects, but quickly found that anything in the effects loop of the JSX colored the tone of the head more than I could tolerate. I dumped the rack stuff and went with good ole stomp boxes which improved my tone signficantly vs. the rack multi-effects. Unfortunately it was at the expense of convenience (ah, the easy days of programmable rack effects with midi control), but it's all about the tone man! I found that the amp sounded so good I used effects more sparingly than I ever had... so the pedals worked out great.
Over time I discovered the versatility of the amp as I started to toy with it for recording. I also expanded my guitar collection to incorporate a couple of Fender Strat's (something I hadn't done in about 10 years) and began playing more blues and classic rock. I discovered that the Ultra channel records very well with the Gain on 3 or 4 using the "Low Gain" input (regardless of what kind of guitar is used). I also discovered that the strat's sound great with the amp through the low gain input and dialing the gain back a bit across the board.
What I've been toying with more recently is revisiting the EQ settings. Knowing that Joe played Marshalls for years a friend pointed out that the JSX might use passive tone circuits (like the old Marshalls did), meaning giving it a go with everything on 10 (ala Jimi Hendrix) might be an interesting experiment. I opened the manual and all the tone controls on the Crunch and Ultra channels are actually active (Clean tone controls are passive). Having tried everything close to 10 I'll tell you right away it doesn't sound good (in my opinion). Also, rolling up the tone controls is similar to the effect of using the High Gain vs. the Low Gain input... you're boosting dB so you'll push more signal through the gain stage of the preamp (which with this amp usually isn't necessary). I was able to add a bit more color to the tone with roughly a T=7, M=7, B=9 setting, but at the expense of having to roll the gain off.
Net, I have to agree this is one of the more versatile, great sounding heads I've heard. In my opinion it's very effective at making far more expensive amps seem like a waste of money. I've taken it to jam sessions with people I've never played with before and I always get oodles of compliments on my tone (even from the members who don't play guitar). Of course I'd rather get compliments on my playing, but I'll take what I can get
I've played through a bunch of different cab's and never been disappointed. My rig is a full stack with the JSX atop B-52 stealth series cab's, loaded with Celestion Vintage 30s.
Here's a link to some material I've recorded with the JSX: