You cannot split the mono out into stereo.
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Not for this. He wants to bi-amp his NHTs and there's no way to get 2 stereo outs for the woofers (not the subwoofer) from a mono source.
The only solution I can think of is to go to an auto/stereo shop and get a pair of speaker-to-line level adapters. But I ain't vouching for the quality of the results.
Your receiver has a signal like a preamp output, which is the input to its power amp. For someone with electronic repair skills, it would probably be a minor job to bring this signal out on a pair of rca jacks, and to provide rca jacks for the signal going in to the power amp. If you were not biamping, you would simply use a short jumper between the output and the input, and many recievers have been built with this configuration. The signal may have a relatively high impedance (because it is not intended to drive a lot of wire) but if you use an electronic crossover close to the receiver it will work fine.
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.
Maybe I'm missing something. If he wants to bi amp why not use a Y cable on the pre out to split the signal to the second amp. Assuming he is drivng the top or bottom of a full range speaker this is a no brainer. If however he wants to drive a second sub then just put the Y on the sub out. In any event get a good Y cable or jumber. I biamp my home theater coming out of a lexicon with no degradation. Hope this helps.