Well, hopefully someone will chime in about your preamp concern. I know many subs offer inline crossover via speaker cables, but hopefully there is a better way.

In regards to subwoofers, value minded I like the HSU VTF-3 MK2, $700 new. http://www.hsuresearch.com/

I have the smaller one, VTF-2 MK2, but recommend the big brother if you can afford the difference. Bass is clean, fast, and flat. In my case my room is the weakest link. But HSU is very helpful in room placement and suggestions. I was able to email a picture of my room with dimensions and HSU made a few recommendations for placement. Also, "crawling for bass" in the How To section is helpful.

How big is your room?