The piezos on my 905EDAF are the only ones I've heard that add frequencies BELOW those of the mag pickup. They sound huge! They add some highs (crisp) too, but the lows were a shocker when I first started messing with them. I just assumed I would turn them off (the piezos). I dont know how they would sound on a fretted one, though. Really, the electronics on these are really too nice to mod, I think. I would just leave it stock, unless you've really fallen in love with the body shape and don't see yourself selling. I have a 905 fretted that somebody butchered the electronics out of, its a project bass for me. Enlarging the pickup cavity will probably be a real bitch- luthite is brittle, and chips like a mother******! and the extreme curves means making a router carriage for your template. I don't think I am going to do this. I did a little research a while ago and I think both Nordstrand and Aero make soapbar pickups taht are very close in size and will probably work. However, this was like two years ago and Aero seems to change their offerings all the time. I'm pretty sure a few companies make something close (smaller) if you're handy enough to make a pickup ring to take up the extra space. Which you would probably want to glue in, since luthite doesn't hold screws very well, either. Bottom line, GREAT basses, but too much of a pain to mod if you don't like the tone. I wouldn't "rip out" the piezos- just turn them off. If you really want to go crazy with different electronics, I would put in a different mag pickup, bypass the onboard preamp, wire directly to the jack and use a beltclip preamp like an Aguilar or Sadowsky. If you're dead set on removing the piezos and really love the bass, do yourself a favor and buy a set of the monorail bridges, and swap the entire assembly. Monorail bridges can be ordered from any Ibanez dealer. Then at least you can sell the whole assembly to somebody (like me)! This will facilitate using an aftermarket onboard preamp, too. Good luck!
PS Use a hardcase- that upper horn will break right off if you're not careful.