River,
I also looked into this a while back. I decided not to pursue, mainly because I didn't want to mess with my speakers, but always thought it was a good idea. This is a good primer: http://sound.westhost.com/bi-amp.htm#separating_sig
To do active biamping right, and assuming you want an amp for your woofers and another amp for mids/highs, you need to take out the speakers' crossover between woofer and mids/tweeter. This is the step I wasn't willing to take.
Then you also need an active crossover between preamp and amps. Rodman's RCS 2.2 is probably one of the best options, but expensive too if you just want a crossover (it does room correction too). Bryston 10-b is another cheaper option. There are also cheaper ones I now cannot recall now. Of course you can have another crossover output for the sub, so triamping in practice, but conceptually the same thing.
I think the above are the right steps to make it work well. I live overseas I cannot get crossovers like this easily, but had I been in the US I would have tried. I did try passively biamping: tube amp with gain adjustment for top, ss for woofer, Rel sub for low bass, and speakers unmodified, and didn't like it better than tubes (MC-275) for the whole bandwidth and Rel sub for low bass. So doing it cheap didn't work for me.
I hope this helps.