The 520 has no problems playing loud and distortion free. Like the above poster said though, keep it well vented if you're playing loud. A thermometer on top of mine read 140 degrees F after an hour session at the -10dB level. Not too worried though.
I just bought the HK avr310, 60watts x 2, 50watts x 5. I had the same worries as you. In a room simmilar to yours, I cranked up an old Billy Idol tune about 65% of the volume control. It was louder than I would usually listen to (very loud!), I don't know exactly what spl. I thought it sounded great with lots more to give. Regarding MarkO's point: Even though I thought the volume was tolerable, My ears started to ache mildly about 2 minutes after I turned this down. It felt like I had been at a loud concert for hours. It went away soon enough, but I learned my lesson.
I've got an AVR320 at home connected to a pair of Infinity Interlude 60's in a 20'x20' room with no 4th wall (open concept). This combo plays plenty loud but it's kind of stealth-like, since there's virtually no distortion. Distortion is fatiguing to your ears while clean sound is not.