Adding a subwoofer to a 2 channel amp

I second the REL, but set the crossover as low as you can and work up. I have my Stadiums set at 22 Hz. The REL provides infra bass, which greatly enhances the realism of your system. It is not a subistute for bad or inadequate bass above 50 Hz.

I would recommend the external crossover (such as Marchand or DbX) and then driving the subwoofer with a separate amp. I have done bi-amping with a second pre-amp out and no cross-over but when the cross-over was introduced there were significant improvements. Building your own subwoofer can result in both significant savings and sound improvement to your system. Many subwoofer amps have built in cross-overs. Check out Parts Express and/or Madisound for possible places to start. Good listening!

I use the RC-972 pre-amp with a Carver M1.0t Main amp.
The speakers are Maneplaners, MMGs, coupled with an HSU research powered sub. The sub has a built in adjustable crossover, phasing switch and volume control. The RC-972 pre-amp has two outputs. I just hook the main to one set and the sub to the other. The system is easy to calibrate and sounds excellant.

I, too have Maggie/HSU setup. Absolutely seamless with very musical bass. My Listening room is about 4000ft3 and the VTF2mk3 fills the bill.
Not bad for flicks, either.