Greetings from the North, This past weekend I was visitng my favorite bass & amp repair shop and I picked up a JJ ECC83S to replace the Sovtek 12AX7WA that comes stock with the XS800H. I'm still infatuated with this head but being an old hot rodder I like to tweak stuff and besides the tube was less than 15 bucks. During a mid break in rehearsal on Saturday I grabbed a screw driver and to the horror of my band mates I opened the top up and and installed the JJ. The room got real quiet cause they just knew that I had wrecked it, until I switched it on and began to play. Great Googly Moogly! Had to jump back like a kid that just poored gasoline on a fire for the first time. No this isn't one of those 'you have to have golden ears to hear the difference' things. The sound difference was very apparent. Richer,louder and more of everything. This thing is now a loud tone monster. To be truthful I didn't think that a pre-amp tube/valve change would make all that much difference,with this head. Especially with something like a readily available JJ ECC83S. Although I read about it in the XS800H long thread, this actually worked and was a pleasant surprise. If you are running one of these I highly recommend this tweak.