No audiophile runs the signal going to the main speakers and their power amp through the high-pass filter/crossover included in a sub. To remove the bass from the speakers and power amp (one of the main reasons for bi-amping, or adding a sub), you instead do one of the following two:
1- Install a high quality active crossover (Pass, First Watt, Bryston, Marchand) between the pre-amp and power amp. The design of these crossovers and the parts used in their implementation (the Pass and First watt are 100% discrete circuits---no ICes, OpAmps, or integrated circuits) are higher than that found in many pre-amps. For the low-pass signal, use the controls on the sub.
2- Install a capacitor and resistor of correct values (so to create a passive high-pass filter at the desired x/o frequency) on the input jacks of the power amp. Perfectionists have been doing this since the 1950’s. This method provides the purest/highest sound quality signal path, but creates only 6dB/octave filtering. Still, better than removing no bass from the speakers and their amp.
