Also, remember that a crossover is not a brick wall. Response will roll off, not stop completely. A friend of mine has 100's (v3's) set at 40 where his SVS B4 takes over nicely. The advantage of nice large mains, as you have, is they will blend ever so sweetly with your sub, whereas a smaller speaker may not be as capable in that respect.

I have my B&W 604's (-3db @ 39hz) set at 60hz, which I find blends nicely with my Hsu STF2. The advantage I found versus my previous smaller towers is the cleaner and more authoratative midbass. The larger tower allowed the sub to blend much better.

Your towers are not wasted with a nice sub. They compliment each other. I play with the 40 and 60hz settings and see what you like.