Warm Tube Amp(s) to Tame Hyper-Accurate Speakers?

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.  

I feel your pain and I mean no disrespect, but if you want to change something, change the way you buy and stop buying without audition. Any other way is just hit and miss. I understand that it is impossible for people in some locations, but I think if you buy new there is usually a return option. That said, I would change the speakers.  These are just not working for you. Been there. Also, while some doubt this, speaker cables made a huge difference in my Harbeths. If your lengths are not too long, try a used set of audience AU24,  or Purist Poseidons, which are not too expensive. Both sounded much smoother and more realistic than Belden cables in my system. 

What is the source of your music? Digital? Analog? That’s worth examining too. I lot of digital streaming equipment ( computers, DACS, servers) sound a bit harsh & over “etched” to me. They can be very tight, clean, fast & dynamic but I can’t listen for more than maybe ten minutes before I need a break. For me listening fatigue is the real determining factor of the quality of a system. Just about every system has something it could do better at but if my brain relaxes I enjoy & get into the music, it’s a “ good” system. 

What about the source? 

That is a warm rig going into speakers that are not known for being bright.  I would need to dig into it, sometimes the wrong impedance curve on a speaker can make a tweeter sound hot when paired with a tube amp.  You might find switching to a neutral SS amp. Naim, AVM, AGD come to mind for me.  You should be able to borrow or buy on an amp on a return policy to test this.  

how is your room?  Damping matters and if the room is underdamped it could be creating this issue.   
 

Finally, If you are looking for a more “English” sound maybe Harbeth or Wilson Benesch will solve the issue.  You ought to be able to find a used pair of used 40.2s or the P2.0s in a similar price range.