Whoops - I guess I've been offline for a couple of days. Jeffreybehr - thanks for your suggestions. In about a month I'll have a preamp that has 2 sets of RCA outputs: 1 with caps that are selected to roll off at 80hz into my 100kohm Cary SLM70 monoblocks, and one without any filters. I will try it both ways: 1) passive filter on the high frequencies and active filter built into the subs, and 2) active filter (Bryston) on both. For now, I can only work with the Bryston, until I get my other preamp.
The sub options include: 1) a pair of TBI Magellan VII's, and 2) a single ACI Titan. The TBI's only have phase reverse whereas the Titan has fully adjustable phase. I'm not looking forward to moving the subs around, but it has to happen! The TBI is very fast, which is why I bought them (to keep up with the Quads). I needed 2 because they aren't that powerful, plus I believe in stereo subs. If the single ACI works, I would likely buy a second.
Thanks all for your comments. Check back on this thread in about a month for a summary of my results (I hope).