PM mo si toybeats... He had the chance to pair B&W 684 htm61 & Denon 2808CI last week. He tried all sort of tweaking, bi-ampling, etc.... Sabi nya bitin daw sa power ang 2808 vs. b&w nya as he can feel the speakers asking for more power. His volume is already set at 0db para lang audible yong dialog. Sayang nga daw kasi malinis tumunog ang 2808 kaya bitin sa B&W 684 nya. 
I find this quite unusual. I have a friend who has a 7.1 channel setup, using JMLab speakers (which can go down to 3 ohms) in a 48 square meter room. He uses a Denon 2808 and I never found the power inadequate. Of course, he has 2 big subs and he sets all speakers to "Small" so that also helps. His system can handle volume at 0db and the dialog is very clear.
I have also heard the same Denon 2808 driving a 7.1 system at the AV Driver listening room (which is huge) and it was really loud.
My guess is that it also depends on speaker positioning, room acoustics, the location of your listening position and if you run the Audyssey setup properly. I find that most of the time, when people complain about inaudible dialog, their center speaker is located on a low shelf, near the floor, and even worse, sandwiched between shelves.
