Marakanetz...I don't know what you mean by the power amp being "disconnected". Of course it is until you feed a signal back in.
Dcconners...If you just want a full range signal to feed into a subwoofer amp that has its own low pass filter, a tap of the signal (without disconnecting it from the power amp) would be even easier that what I suggested. However, this would leave the receiver's power amp, and associated speakers, running full range, which doesn't really gain the full benefit of biamping. Also, most subwoofer "plate" amps have provision to input a speaker level signal. If you went that way no changes would be needed to the receiver.