I am experiencing a like problem. In my case the "listener fatigue" symptom came on like flipping a switch. No changes to my rig.
Fingers are crossed I have had a E/N/T infection that became worse, so it is to the Audiologist for me as soon as I am convinced I am feeling better.
How am I sure of this? After my system started sounding like poop I have tried everything to solve it through DSP/EQ via Roon. Days of going through all the possible offending frequencies, speaker placement... even the old Yamaha NS10 trick.... gauze over the tweeters lol!
I will give David Hafler (RIP) his props. I purchased a DH220 in 1981, another in 1985 which is in use now. Both amps sounded great! I had the 1985 upgraded in 1989 by Musical Concepts. Sounded better all the way around.
A little over a year ago it was upgraded again with all Fantasia Audio boards, caps. All jacks and speaker terminals replaced with Cardas. Morrow Level 3 power cable hardwired. Total investment since 1985.... $1500.00, $41.00 a YEAR! Oh, and in my configuration of nearfield, 1M w/85dB Elac's, I am locked into Class A only using Nelsons most important First Watt moniker... If the first watt doesn't sound good lol.
I mention this as I am all Morrow level 3, except some power cables. Way broken in by their 400+ hour reccomendation.
Good luck!