If you are looking to dial out the mid spike, then tubes will not completely do it. The JSX is voiced so that the preamp rolls off everything below 1.0K. So basically the preamp is really emphasizing the mids. The active EQ after the preamp does allow you to add bass, but it is like trying to EQ out a cocked wah, which you really can't do. The "FAT" switch should be called the mid spike switch, with it off, all the lows below 1.0K are rolled off. With it on, all the lows below 600hz are rolled off (thus adding some bass.) but still emphasizing the mids.
Since I design and build my own tube amps. I went ahead and modded my JSX so that the mis spike was no longer there, and it sounds much better with any of the aforementioned power tubes.
The JSX is really an awesome platform to do some serous easy mods, to really make it a monster amp.