Before any meaningful "solutions" can be offered, we need to understand the problem. First, saying the mids and highs are too much could mean the same as the bass and upper bass is too thin--after all, tonal qualities are about relative balance of frequencies. Is this a case of someone wanting more bass or warm in the upper bass rather than having issues with mids and highs?\
Could there actually be a problem with the gear or set up? Has he had anyone else listen to diagnose if there is something completely out of place? Has he tried any other gear?
Is this a matter of particular taste? It would be helpful if the OP were to offer comparisons with other systems he has heard and liked or disliked.
I have heard the MC275 VI and did not like it at all, but not really because it sounded bright--mainly because it sounded lifeless and unengaging. It did have a slightly hard and brittle sound, not unlike other KT88 amps, and perhaps this is the issue.