Good choices in a used A/V pre-amp?

I'm slowly re-building a home theater. My amplification is about the only thing set -- a B&K TX4430 (200w x 3) for the fronts, and a B&K EX442 (200w x 2) for the rears.

Now I need a DTS pre-amp, and I don't feel any need for 7.1 (or should I?). I'd like to try to stay around $500 or under, hence going used. I haven't made any firm decisions in the speakers I'll be driving, a project for another day. This is a home theater system only -- no music listening. I'm re-building because I'd broken up my rig long ago for a dedicated 2-channel high end system for another room.

Is my $500 price point enough for a used a/v pre-amp? What are some key features I should look for? Brands and models?

Thanks!