I've not seen anyone else suggest that a third bookshelf speaker would be superior to the speaker specifically designed for the center. Can anyone else comment on this?
As for the towers, I was reading up on the forums and I found quite a number of audiophiles (maybe even the majority) preferred the bookshelf/sub setup over the tower setup. The reason for this, of course, is that, when paired with a nice sub, you can get better frequency isolation and the setup can be more finely tuned. I already have them paired with a great sub that I can't even turn up past 30% volume without rattling the crap out of my cabinet. Now I just need a center channel to match the quality of what I already have.
I should also mention that I'm much more interested in quality and clarity of sound reproduction than in volume. I listen to a lot of music and i"m more interested in having the rich, warm sound of acoustic instruments than in rattling the walls with electronic bass.