Hi,
Not that I can exactly 'corroborate' your problem, but I do experience a related 'issue' with longer periods of listening. Initially, the soundquality coming out of my system is heavenly. And per definition always a break from my other ongoings and their related mindset.
And then, after say half an hour, I tend to start hearing a different sound quality, or at least interpret it differently. At first I found that annoying, (in fact I still do) and started searching for a reason, and of course a solution.... Tried all individual elements in the system, and I can not hide the fact that the purchase of the new BHA1 was partially justified because of that. I even tried my old kef102's to replace the ESL63's, and in that mind-set (or should i say ear-set) at that time I felt they sounded better

All elements seem to be perfect. The BHA1 is heavenly, as is the rest of the system.But even with the BHA1 after a longer period, my ears 'hear' differently. Rules out possible issues with my ESL and pre-and pro amps. Will try the ModelT as soon as it gets here though

Summing up, I must believe the culprit is our own Mind(set), and possibly our hearing mechanism. Because of that, I read some psychological theories pointing in that direction, but right now can not find their links, sorry.
I think it was comparable with the well known effect of looking at a red picture for a minute, and then closing your eyes, or even more evident, a white paper. Your brains show you the opposing component color belonging to the red picture. Apparently something like that is happening in experiencing sounds also. Experience a different sound when closing your eyes? Or try this: like a dominant eye, you might have a dominant ear. There's all kind of situations where we think we do things 'stereo' but in fact point one of our sensors in a more prominent way, depending on our complete physical state of that moment. Humans are the variable here

The other day, I was at a concert where Reich's WoodenMusic was performed. After 15 minutes of clicking Claves, the applause sounded completely weird in my head, up to the point I felt a dizzyness coming up.
Trust your B135, even your Speakers, but always doubt your own ears and mind....... You're not alone
Good luck,
Marius