Cross-overs can do this. I have a Coustic 5e (or something like that) which I bought for those exact purposes but ended up using the internal crossovres of my amps instead. I didn't think the 20x4 amp came w/ them, hence why I bought the cross-over in the first place.

The Coustic has connections for remote front-center and rear-sub volume controls in addition to typical crossovers for both the front and rear.