Actually, you may not want a volume control on the pre out from your Jadis. Is this a preamp or integrated? It's best if you eliminate cascading volume controls and crossovers along a single signal path. Let the crossover control the volume. The only possible problem that might pop up is if the output impedance of the integrated and input impedance of your subwoofer amp aren't well matched. I experienced this once with an Electrocompaniet ECI-3 integrated and NHT SA-3 sub amp. No matter how I adjusted the volume , crossover or phase settings on the NHT amp, I couldn't get it to sound right with the ECI-3. This isn't very common though.
Choosing which sub to build and what size driver to use depends a lot on the what you want it to accomplish and the size of your room. For music I definitely wouldn't go too big. No more than say 10" in a small room and 12" in a medium sized room. If the driver is too large you'll sacrifice speed and detail, and it may be more difficult to integrate with your main speakers as a large sub trying to keep pace with small, fast speakers won't synch well.
Choosing which sub to build and what size driver to use depends a lot on the what you want it to accomplish and the size of your room. For music I definitely wouldn't go too big. No more than say 10" in a small room and 12" in a medium sized room. If the driver is too large you'll sacrifice speed and detail, and it may be more difficult to integrate with your main speakers as a large sub trying to keep pace with small, fast speakers won't synch well.