Usually you are better served with a single 'larger' amp driving the whole passive speaker. So if you were purchasing amplifiers you are better off with a single 4B rather than 2-3B's.
If you have an amp available to use on the tweeters mids tweeter it certainly will not hurt but I just want to inform you that the differences will not be great.
I'm no audio expert, but my previous experiments with bi-amping agree with James' comments. I have tinkered with passively bi-amping B&W 604S2's using an 8B-ST. I found it was better than single-amping with a 3B-ST - there seemed to be less distortion at higher levels. I had a 4B-ST in the closet that I had never used because I felt the 3B-ST and 8B-ST should have more than enough power. One day I figured that I should at least connect it up to make sure that it worked since I had bought it used. I didn't like the 4B-ST at first because it seemed to roll-off the high-end compared to the 3B-ST. The 4B-ST did have a lot more bass though. After talking to James, I realized that the frequency response of the 4B-St was smoother than the 3B-ST so the high frequencies of the 3B-ST were actually more apparent because of a poorer frequency response at low frequencies. To sum up, I like the 4B-ST a lot more than the 3B-ST.
I've since done single-amping tests between the 3B-St, 4B-ST, Citation 5.1, Citation 7.1 amps, and B&W 604s2's, B&W 600s3's, B&W 601S2, and PMC IB1's. I always preferred the sound of the 4B-ST. I can't explain why because I don't know the audio lingo (bought my first stereo 3.5 years ago). It just sounds better to me.
Sometime in the near future (meaning hopefully before xmas), I still intend to try bi-amping/tri-amping my IB1's with various combinations. I imagine that I will probably eventually go with a pair of 7B-ST's (single-amping) for my IB1's, however experimenting is fun. If the difference (to my ears) between the 4B-St and the 3B-St, is as good as the difference between the 4B-ST and the 7B-ST, then it will be well worth it.
editted August 20, 2003
"Is anybody else out there using 7B-ST/SST or 14B-ST/SST with their PMC IB1's ?"
Is anybody using 7B-ST/SST or 14B-ST/SST amps with their PMC IB1's ?