I have been looking into the same with speakers and subs especially external cross-overs and a designer friend of mine offers a few points on what to look for on the subject on cross-overs, he makes a good point on integration. He likes the Marchand equipment for the pirce. When discussing the controls he wanted me to look for ones that have phase and delay controls for subs or woofer towers to time-align with the main speakers.
Subs with active crossover
I have some monitors I like a lot (Meadowlark Vireos), but they are not designed for the deep bass in the electronic, ambient and film soundtracks that I listen to. I'd like to hear about Agon members' experience with subs that use active crossovers to filter out frequencies below, say, 60 hz. I know REL is the favored brand, but am curious to hear about other (maybe cheaper) brands. Are there other subs out there that can do this with no audible impact on sound quality from the mains? And will this always require connecting my speaker wires to the sub?