i've played the jsx and xxx quite a bit, and for me the jsx is the winner there hands down...obviously i don't own either of them tho, not because they're bad amps, but because they just don't interest me that much
some people just don't like the 5150 plain and simple, and one thing i've learned is that you can never put too much stock into clips and opinions from other people...an amp is only as good as it sounds when you're plugged into it; you can chalk it up to bad tubes, wrong cabinet/speakers, wrong guitar/pickups, operator error, etc...but if an amp doesn't sound good in your possession then it's not the right one for you, no matter how many people think it sounds great
for the longest time i wanted to try a vht cl100, i heard a lot of great clips of it, talked to a lot of guys who'd owned some of the same amps i did, by all means i should have loved it; when i finally got to play one it really didn't do much for me...then on a whim i bought a vht deliverance not expecting it to be what i really wanted, but just curious, and i got it for a good enough price that i easily could have re-sold it for profit; and almost a year later i'm still shocked at how much i love the amp
i wouldn't get too nervous about problems with the 6505 tho, i suppose it's possible they've got a few out there with bugs now, but overall those things are tanks...you would not believe the hell my first 5150 went thru (i was the 3rd owner, and knew the previous two) and the guy who bought it from me almost 4 years ago still loves it