AVRs are almost all rated with 2 channels driven. You are going to lose 20-40 watts right off the top with a 5.1 setup. You can help the cause by setting your speakers at their -3dB point (about the lowest freq they can adequately produce, not their full frequency range) or higher.
If you are using an adequate sub, then your mains shouldn't need to be less than 40 - 60 Hz except in two channel or pure audio mode. The Onkyo disables all of the equalizer and sound settings for the two fronts in pure audio anyway.
Honestly, I hear practically no difference with my XPA-2 at low to middle volume levels. Heck, the Emo doesn't even bat an eye during movies when in Dolby Digital surround mode so they're not using much power at all. When playing music in pure audio or stereo 2.1, they rock from about 65 - 90. If you want loud, crystal clear, pure, sharp, and accurate sound at higher than normal volume levels (reference levels) you need an external amp. That is where my XPA-2 really shines. The Onkyo NR3009 just couldn't keep up with the bass demands of the RTi A9s at anything above 60 - 65, and it has a massive high current power supply and 140 WPC (2-channels driven).