bass management with integrated amp

I have a classe cap-101 integrated amp driving paradigm studio 20's v.3. I'm missing the bottom octave and would like to work a sub into my system. I'd also like to use some kind of crossover to relieve my smallish speakers from the lower end of the signal. My problem is that I can't figure out any way to do this, since my integrated can be used as a pre, but not as an amp (pre-outs, but no pre-ins). Any suggestions would be appreciated. Also, I would like your suggestions for musical subs in the same price range as my speakers and integrated.

Wish I had found a'gon sooner, bought my gear used, and thought ahead...

Have you listened to any of the subs from paragidm? They might do exactly what you are looking for. Also, I think you can throw the X-30 crossover from paradigm in front of the sub using the line out on the preamp. That might be redundant though, I'm not sure that is the best idea. Try running your 20's full range with a sub, you might like the results. I've had three pairs of studio 20's and have always run them full range. Another sub to look into would be a Sunfire True sub from Bob carver. A small little brick, but if set up properly, can sound pretty amazing.
Thanks. I was thinking about both of these brands of subs, and also running them from the line out on the preamp. I'm happy with the studio 20's, but wondered if they would open up further if I freed them of bass duty below, say 60 or 80Hz.

Another question, if I may. With a fairly small room and living in an apartment, I don't need to go loud with a sub, but deep would be nice, and most of all, fast and tight. The goal is to get nice low end at low volumes. Do I need a servo-15 for that, or would a smaller sub do as well (or even better, given my room)? Or would I better off getting full range speakers?

For most of my listening choices -- classical and jazz mainly, some blues, classic rock and alternative, but mostly piano and vocal classical music -- the 20's are fine, but I'm not getting the richness of sound I'm looking for.