Well, FWIW, I have mild tinitus and I can't abide any irregularity or excessive brightness in my audio. This has driven me to pay close attention to eliminating these problems. The result is I can listen for hours w/out difficulty. In fact in tuning my system as I have, it invites listening at loud level just because the extra volume can expose more detail and draw you further into the music, without 'hardening' the upper frequencies. And in my room with my stuff, I have excellent soundstaging including depth of image, a hallmark of good resolution so I'm not just rolling off the high end to get a dullish result. Its really about removing or reducing distortions from room/set up and your equipment.
I have done this by using tubes in all my equipment except my tuner so I can tailor components output to fit my needs. It is just amazing the difference tubes can make, its not minor at all. I'm not suggesting that the tubes in your Manley are your problem, not at all! In fact it could be something a simple as the output taps you are using on your Manley.
Interestingly, I recently acquired some efficient speakers with a nominal impedence of 4 ohms, a minimum impedence of 3.5 ohms which sound much better off the 8 ohm taps on 4 different amps. Who would have thought! Took me a couple of months to figure that out - I was blaming everything else for a problem that this change solved.
Re tubes for example, I tried a pair of tubes in the input stage of an amp. The tubes initially sounded full and detailed, until I played a recorder track on a test disc. A tough instrument. In its highest registers it caused a resonance in the tube which caused a very unpleasant (unlistenable) band of distortion. When I listened more closely I could hear this distortion to a much lesser degree on normal highs from music other than a recorder and it was just unplesant, but much harder to identify. Put in a different tube - resonance and unpleasant sound gone. Don't write off the effect the different tubes can make for the better or worse.
Another thing to consider is whether or not your amp and your speakers are a good match. Once again, considering my amps which range from 35 to 160 wts, the 35 watt amp is competent on paper to drive my speakers, however at loud levels (90+ db's) the upper mids and highs harden up and it becomes unpleasant. Not a problem in the 80+db range. Not a problem with my other amps.
Another thing to consider - its easy to overdrive a room the size of yours. If your equipment is the source of your problem you should hear the problems at medium volumes. If you only hear the problems at high levels you could well be overdriving your room - you have those high frequencies endlessly bouncing around. Room treatments can help with that problem to some degree.
Lastly, humor me for a minute. Try crossing the axis of your speakers well in front of your listening position and see what happens. Some interesting things go on when you do this, not all of them bad! You can get a more focused center image, a wider sweet spot, and minimal first reflections from the side wall.
Just some food for thought. Hope it might help you a bit.
