I've heard of people putting the Tripoles in the rear - with Tripoles alread at the sides. M&K does make a "Tripole" speaker that's essentially a 150THX. It was initially a pro model (check the pro section of their website), but I hear they're releasing a similar (identical?) consumer model. Could be a good alternative. I'm currently using three 150THX's up front, Tripoles at the sides, and S125's in the rear. I'm thinking of possibily upgrading the rear S125's with another pair of 150THX's, while keeping my Tripoles at the sides. BTW, I run a Lexicon MC-1 in full 7.1 channel mode. What processor are you running?
I see you're in White Bear Lake. I'm in Minneapolis. Would be interested to hear more about your system.