It sounds likely that the amplifier is starting to clip at that point. Clipping causes a rapid increase in distortion such as you describe. Try backing the volume off just a little bit at a time until you get to the point where this no longer happens.
"Sizzling" tweeters at high volume?
I have my new SFC-1 tube amp (Svetlana El34) 50wpc. my speakers are ACI Sapphire II (Focal TK-120 kevlar tweeters)rated 15-150wpc, 90db with imp. of 6ohm. On some recordings (Live at the Greek, Tim Haggans...)snare drums, and trumpet would start to "crackle", at volume level past 12 o'clock. On other CD's it doesn't(Kind of Blue)