what's the rail voltage? the reason i ask is that one of the "rules of thumb" for power amps is that the power amp clips at between 1 and 1.5Vrms input (standard line levels used in home audio). so for instance, a 100W amplifier clips at 30Vrms out, so it's set for a voltage gain of 30. so the 100W amp (assuming an 8 ohm load)would have an input waveform with a peak value of 1.414V, and the output waveform with a peak value of 42.42V, so the rail voltage would be about +/-45V or more (many manufacturers use +/-50V). an LM3886 is a 75W amp, so for an 8 ohm load you would use +/- 40V rails with a clip voltage of about 35V (it will actually be a little higher). this is an RMS voltage of 24V, and again an input voltage of 1V, so you might actually want R701 to be 24K.