IIRC, I think the tweeter is like $25. I could be way off. My dealer told me the bass/mid drivers are like $75, and that's at his cost.
Just contact PSB directly. I've had good experiences when contacting them. You can also just try though a local dealer if you wish.
If you ever want more T55s or a C60 at some point in the relatively near future, lemme know. I've solicited a couple of folks here, but I won't put up any ad until I get around to saving my money for speaker parts to build a new three, let alone successfully building them. I have two pairs of T55s, and one of those pairs I'd sell for very cheap because it was a repair project (damaged on way to dealer, so I took them off his hands at his cost with the understanding that I get any needed parts as free under warranty). You could have spare drivers galore haha, though admittedly you'd have to blow at least a couple of drivers to get your money's worth.
If you want to see some internal and bass driver shots of your speakers, here ya go. Please forgive my absolutely non existent photography skills.
http://forums.audioholics.com/forums/showthread.php?t=68668
edit: the repaired speaker above is acting as my new (and temporary) center speaker behind my DIY AT screen.
IMO, one of the more prominent attributes of these speakers are the ability to crank, at least as a subjective impression regarding price point competition. You might consider adding a very affordable 2 or 3 ch amp to your 663 if you want to push them. I have the feeling they can handle more power than what a 663 can cleanly give.
