If you do hear a difference, try to define what it is... dynamics, frequency response, distortion, etc. If it is frequency response, see if you can correct for it using the Onkyo's tone controls. The mono block amps may have a "house sound" coloration. It isn't uncommon in amps without tone controls.
What I did hear without the blind testing and level matching was that the Quads were alot more dynamic, not in the volume sense but in the way drums for example were upfront and very clear sounding. On big band recordings such as some Mingus/Ellington to name a couple, instrument seperation was better and the presentation was overall much more airy. The level of detail for the most part was a few steps above the Onkyo, so I'm guessing frequency response was also a factor. The Onkyo sound was more muddled with a lot less "space" if that makes sense.
I really dislike reviewing gear because honestly it sounds kinda silly using words like 'airy' and 'muddled' to describe sound, but for lack of a better vocabulary, I'll have to stick with what comes to mind first.