Rogue Atlas Magnum sounds bright -- any recommendations?

Hifiman,
I have the Platinum-upgraded version of the DNA .05, but when I recently heard Don Sachs' KT 88 amp, I was flummoxed... it has vastly more three-dimensional sound, no question, and a potent LF response, that did not expect in comparison to the McCormack amp.  

I ordered some NOS GE 5751 tubes to swap out for the stock 12AX7's to see if that tames the brightness I am perceiving.  I have little room to move here other than swapping input tubes.  It is quite a fine amp, but for the brightness I am hearing.  

Gents,
I have spoken with Mark at Rogue and he is very helpful, as you suggest.  Mechans, I hear you on the input tubes and have some nice NOS 12au7 and 12ax7 tubes to roll through the amp.  I have Cerious Technology GE IC's in my rig so I will live with them.  

I have recently heard amazing tube amplification with Don Sach's KT88 amp in my system -- it did everything better than my upgraded McCormack amp.  The Rogue amp is not, to my ears, in the same league as the Sachs amp.  KT90's are perhaps are perhaps not as linear as the better KT88's.

Thanks again for all your helpful suggestions, they are all appreciated.

I just changed the taps from 4 ohm to 8 ohm and the music sounds very thin.  With my 11 ohm speakers, I thought this might be helpful, but it is a move in the wrong direction.  It was worth trying. 

Lowrider,
I have a ton of tubes with poor markings on them so I gotta get them all to my  tube tech to ID them properly and test them.  I did swap some Mullard NOS 12AX7's in the amp and they seems to take the brightness down a fair bit.  The NOS GE 5752 tubes might also help.  

You are right to suggest swapping the KT90's out for KT88's or KT120, both of which I can borrow from pal.  They are all compatible with the amp.  I forget that the curse and blessing of tube amps is rolling tubes.  The NOS tubes are pricey.  Thanks for the help, my friend.  

JohnnyR,
Thanks for the encouragement.  I have talked to Rogue about an upgrade, which is affordable, but my experience with upgrading a boatload of audio gear, is that they pretty much just works around the edges and one can't get a nickel out the costs of the upgrade in the aftermarket.  I am gonna roll some KT88's and KT120 through it see if it sorts out the HF sibilance I am hearing.  The amp does a hell of a lot, but the hard edge of vocals is a show-stopper for my ears.  Cheers.