Adding a passive sub system Help.

I would like to add a pasive subwoofer to my system Muse model 1 preamp, hafler p7000 amplifier (350 watts at 8 ohms per side), Infinity rs 6 kappa speakers. So I have some power to spare but only two speaker outputs and crossover quetions. Thanks for any help,
I'd contact Kevin at Muse for his recommendations. BTW I use an old Janis system for my HT system. If you can pick one up cheap it might fit the bill.
Why not active? You gain much flexibility in my book with an active sub system. The ability to fine-tune the level for one, the phase for one, and the crossover for three. . . .
Active sub sounds like the way to go. Thanks for the help