I second Peterayer's comments. And I'm surprised that the thread reached nearly 40 posts before the word "loading" was mentioned.
The first things that come to my mind upon reading the description of the symptoms are
overshoot and
ringing. I see that the EAR 88PB phono stage/preamp uses SUT's at its inputs. Which suggests to me that a loading mismatch could cause the response of the cartridge or the transformer or both to overshoot and ring in response to high speed transients, overemphasizing them unnaturally and causing them to be fatiguing. Or if the ringing is at ultrasonic frequencies, it may cause side-effects in downstream circuitry that are at audible frequencies. And as Pete suggested, comparable effects could conceivably result from other causes at other places in the signal path.
Not sure what specific course of action to suggest, but those are some thoughts to consider.
Regards,
-- Al