The KT88, 6550, KT77, KT66, EL34 all fall under the "more" category. I'm afraid you're pretty much stuck with 6L6, 5881, and 6V6 on the low end.
http://tdsl.duncanamps.com/tubesearch.php
As far as adding an FX loop goes, there really isn't a good place to throw in a passive loop like we did on the VJr, and there isn't a typical cathode follower stage for properly driving a loop unless you want to make your own. If you want to add the tube (or 3SB board) and circuitry for a driver and recovery stage, I'd recommend inserting it before the reverb send.
A Vibrato circuit is gonna be yet another tube. Honestly, if you want an AB763, ya might as well just gut it and start fresh. Or keep it for what it is and get to butchering a fresh chassis.
Gil...