I don't know for sure if the 635 is comparable to the 110 watt a channel Yamaha and Denon, but I do know that it has no problems supplying enough power to Monitor Audio RS 8's( power handling RMS 175 W ) with watts to spare.I was told I needed at least 100 watt per channel reciever to feed the MA RS 8's.After purchassing the MA RS 8's and RS LCR , I hooked them up to my HK 65 before decidding on my new reciever. The 65 produced enough power that my origional replacement choice , the 635, was a slam dunk.The 635 is a wonderful step up from my HK 65, but I can't knock the Denon or Yamaha because I haven't compared.
As Bill pointed out in his real world experience, H/K makes a powerfull receiver. They also handle lower impedences well, which is an even better indication of it's real power. I don't know which specific Denon and Yamaha models you're referring to, but in general I would take H/K over the others any day. Denon's notoriously don't handle low impedences well; even though your speakers may be rated at 8 ohms, they will most likely dip down significantly lower. Yamaha is know for over inflating power ratings. I would say (again, I don't know which specific models you're referring to) the H/K's true power is at least equivilent, if not better than the others.
If you get a chance to compare those 3 brands in surround mode, try the Logic-7 from HK. The 75 watts thing might not mean so much afterwards. Speaker sensitivity makes a difference too. A personal favorite would be the Infinity Beta series as they start at 90db.