I tracked back through your posts and see you have some Bose speakers. Unfortunately Bose doesn't disclose the speaker's sensitivity. I would guess it to be around 90dB 1 watt/ 1 meter. This means that the speakers can achieve 90 decibels of output with 1 watt at a distance of 1 meter. For every increase in 3 decibels the wattage must double. To achieve 93dB the wattage would need to be 2 watts, 96 dB would require 4 watts, 99dB would require 8 watts, 102dB would require 16 watts, 105dB - 32 watts, 108dB - 64 watts, 111dB - 128 watts. Realistically your speakers won't be needing that much power on a constant basis, otherwise you'd damage your hearing. A receiver rated 100 watts per channel may have peak outputs up to 150 watts or more for a particular channel for headroom.
Granted you have two pair of speakers connected to the same Yamaha receiver at present, so the output will be diminished for two reasons. You are trying to divide the power for 1 pair of speakers among 2 pair and the combined impedance is halved making the amplifier work harder to achieve the same output level of power. Your efficiency is effectively 1/4 of what it would be with just one pair of speakers connected to the receiver. The receiver you have, and most other stereo receivers, have those A and B speaker connections for the benefit of having two pair of speakers connected, but not to run simultaneously. Because stereo receivers like yours are not Quadrophonic (receivers from the 70's that actually had 4 dedicated channels) they are not meant to power more than a pair of speakers at any particular time. That doesn't mean it can't power 2 pair of speakers, it just means it will perform poorly when asked to do it.
Your other option is to get a multichannel home theater receiver. Most modern home theater receivers have 5-7 on board dedicated channels of equal power and have a sound mode called 5 stereo or 7 stereo. I would suggest looking into a budget home theater receiver for what you have.
Since you expressed interest in making some subwoofers and using a large power amp to drive them I'd suggest looking for a multichannel receiver with preouts like the Marantz receivers offered on Accessories4less.com. The refurbished models offer a good value. If HDMI isn't required and you want to save even more money, then keep an eye on ebay for a used Pioneer VSX-815 or VSX-816. They are inexpensive receivers with respectable power output to sufficiently drive your Bose speakers and they have preouts.
