Hey Dan: I understand that the OP has an 8", 100 W - 200 W subwoofer that has a frequency response of ~ 27 Hz +/-3dB. This depends on which model it is. My take is that by upgrading the subwoofer, mirage1000 can reproduce frequencies that he/she is not currently able to replicate. This would also take the burden of the power-hungry lower octaves off the AVR and shift the 80Hz and below frequencies -- which require the most power -- to a dedicated driver and its accompanying AMPLIFIER.
Adding an a 200-300 W/channel external amp and cables, like the Emotiva or Outlaw you recommend, will probably only yield 2-3 W increase over the 125 W/channel AVR currently used. An amp may not even be needed for a room with unknown dimensions to achieve reference levels and its addition will still not reproduce the lower octaves of the OP's 100% home theater requirement. Not to mention, a 200 W amplifier alone will cost around $900+. In my opinion, this would be better spent on a good subwoofer and could provide a greater frequency response as well as the benefit of a dedicated power amplifier for a lower distortion 15" driver.
Power Sound Audio
Amplifier Power: 500W RMS (continuous), 1000W Peak (dynamic)
This is about 2x the power of the amps suggested, or 4x the power of the Op's AVR.
Here's a simple SPL calculator that can help determine if an amplifier is needed or required.