Clean electric current
Guys,
I have the Polk RTi A1 and experienced the same listening fatigue, there was somethin unbearably harsh and chalk-ish in the trebles.
Those effects were most noticeable with my Virtue Two.2. When hooked up to my peachtree iDecco alone the treble was less agressive, maybe just because it resolves less than the Virtue.
However the real problem with overly agressive treble can also come from your current. I listen to my gear in my office/studio which is a room in a larger office loft space with a bunch of electronics plugged onto the grid. I somewhat always suspected the current to not be very clean, I think it is a very noisy electrical environment.
So I bought one of Supra's Shielded Mains Blocks which applies some mild filtering to the current, taking the jaggies/spikes out of the current to make it 'sweeter'. Opted for the Supras because they are very good value and it's a 'no bullshit brand' (my loudspeaker cables are also from them and these alone sound fantastic and very transparent, better than some more expensive Synergistic Research ones I tried).
Result : The speakers now sound very noticeably sweeter in the highs, never agressive and fatiguing, just more 'liquid' and pleasing.
So, if you experience agressive and fatiguing highs it can also be related to your current.
Best,
Philippe