Good to see you completed the Theatre-4 and you're enjoying it. They don't go all the way down to shake your walls, but low enough for most music.

I'm assuming what you want to do is keep the passive crossover in your Theatre-4 and use a 2-way active crossover to integrate a sub. There's a couple of options:

1) Active X0/Biamp. In this option, you'll need a 2-way active crossover with a summing function for the low freq, and a monoblock amp in addition to your Paramours. Sequence will be CDP -> Foreplay -> Active XO. One output from the XO will go to Paramours than Theatre-4. The second output from the XO will go to your new amp than sub.

2) Plate-amp/subwoofer amp. While the first option is technically the "best", it doesn't have to be that complicated nor expensive. A lot of us that received great results from a simple plate amp. All plate amp and subwoofer amps have an active low pass filter built in, so you don't need a separate active crossover. Just build your sub, slap a plate-amp on it and connect it to the output of your Foreplay. You'll have to split the output of your Foreplay so one goes to the plate-amp while the other connects to the Param's.

If you're looking for something a little different in a sub, Bill Fitz has a couple of interesting options on his site.

Gar.