OP
When you wrote that as the volume increases, the Treble starts to sound shrill. I have encountered this, and the culprit is that the AMP cannot give the power to the speakers. when we replace the amp, problem solved. I have owned a couple of Rotel Power amps, and they do sound good, but they all seemed to run out of steam as the volume comes up. even driving horn speakers
Being cost sensitive, I suggest used. a Used Pass or Luxman may be the fix.
I have seen good reviews of the Litton so that is ok for now. I truly suggest a powered sub like the RELs and SVS line. BUT I truly believe that the possible culprit is that your amp is running out of steam as you punch in the power