I ran into this first hand. With my Hafler DH-500 at 255WPC I knew it was not distorting, yet it sounded terrible when I increased the volume. Also the sound would change drastically at different parts of the room.
Bought accoustic panels, the OEM version of owens corning 703, ~$50 for 6 panels, 2'x4'x2"(exact same product, no brand name on box, so much cheaper) at the local spi-co dot com office.
Placed most of them behind the speakers using the DELE method (Dead End Live End). Instantly the volume level dropped, but the sound became much cleaner, because before I was hearing the same sound multiple times as it bounced around the room. Previously I could not reach 0db on my pre, now I was easily getting there and wanting more.
Yes, it looks like crap, but no WAF, and everbody is amazed at the sound. Need to cover the panels in fabric. Best tweak for the money, no doubt. Also now I hear much more detail, including other things vibrating, windows, the TV, but that's another story.
BTW I had to upgrade the speakers before I damaged them. They were only rated at 100wpc (B&W DM602s) and they don't care about how it sounds, only about how much current I'm driving through them.
Regards,