I'm definately in the camp for running a speaker like your Proac full range. -- As to your question, however, my experience is that to get a good match, you need cross-over frequency, volume, and continuously variable phase control for a sub. I've never heard the piece myself, but another thread identified the Paradigm X-30 as a good control unit. It has low-pass filter, volume, and phase control for the sub and if you want to try the high pass filter approach for your Proac it has one selectable at 50, 80, and 120 Hz. Good luck.
Subwoofer Blending
I am considering buying a REL or Velodyne (or similar)subwoofer and want to "cut off" my Proac 2.5s at 50-60 HZ, blending in the subwoofer. I want to ease the burden on my SET and also get better bass response. I know most people recommend running speakers and subwoofer full range, I don't want to do this. Will I need an outboard electronic x-over or these products have built in x-overs for the purpose? Does my pre-amp need a special input for connection, would I just be able to use a line level input?