Rogue Atlas Magnum sounds bright -- any recommendations?

Gents, I need some help here.  I am making a move back to a tube amp, in place of my McCormack DNA .05.  I just acquired a early version of the Atlas Magnum with KT90's and it sounds excessively bright to my ears.  It has stock tubes.  I have a Don Sachs preamp driving my Spatial Audio M4 Triode Masters.  

I need to tame this brightness to better enjoy this amp.  Thanks for any input you can provide.   
Any particular reason you're taking the DNA .05 out of your system?  As SS amps go, the .05 was well regarded for being a smooth performer rendering fine detail.

If you really want to max. out the .05 you could call Steve McCormack at SMc Audio to see what he could do to update it.  I recently had my DNA-1 upgraded to Gold Special status and it is fabulous.  Not a lot of 💵 for the extensive upgrade.
Both the Atlas Magnum with KT120 or the earlier review with EL34 had High frequency problems as far as bench tests went. Maybe your hearing this.

" Though I must admit to some disappointment with the amplifier’s lack of high-frequency linearity, in this respect it is no worse than the original EL34-equipped Atlas"

" I wonder why a touch more negative feedback was not used to make the high-frequency performance more linear"

Cheers George
As Ralph Karsten (atmasphere) has been saying, distortion is perceived as tonality. Less high frequency distortion = less brightness.

Less high frequency distortion = less brightness.

Correct more air and transparency. This goes for the tweeters also.

Have you ever heard a plasma tweeter? The least distortion of any HF driver, because the plasma flame is the diaphragm and it has no mass, the highs just dance in front of you in 3d space, like fireflies that you reach out and touch. next in line are ESL’s and then so-on down the line.

Cheers George

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.  
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I would definitely say try rolling tubes and can the amp use power tubes besides the KT90s?
George, I've never heard (or even seen) a plasma tweeter, so an ESL or ribbon driven by tubes is as good as I've heard. Transparent, grain-free purity, but only if the amp is capable of producing high frequencies at low distortion. As the OP has learned, not all are.
I would ask Rogue if your amp could safely use the KT120 tubes.  I had a Cronus Magnum integrated with 2 NOS Mullard CV4003 input tubes in place of the 12au7 tubes and the Tung Sol KT120 output tubes and it sounded very smooth, not bright.  You could use EL34 output tubes and they will give you a more romantic sound.  You would need some serious bias adjustment, probably 3 full turns of the control to bias correctly and Roge told me it is normal.
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. 
I don't know if adding $4500 worthy of massless tweeters will do you good, but here are some tone controls, I found very effective. Please don't throw things at me for this.  I had a bright analytical system with tubes and Focal Electra speakers. I introduced Cardas ICs Golden somethings and it was like heaven.  Second I highly recommend rolling every tube I had KT-90s in a small amp and found them bright as well, use GL reissue KT77s.  Get every small signal tube that is Chinese or old Russian out.   Replace with NOS if possible or GL reissue (yes I know they are Russian but they are mellow).  Get the cables I kid you not they are now NOS or used but that makes them cheaper.
@mechans  +1. Everything I've ever read about the Cardas Golden cables indicates that they were well named.  I can't speak to the tube situation but what you recommend makes sense.

OP - Agree with @donjr  give Rogue a call.  No one knows their gear better than they do.
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.
Back off on the Bias
  Install some new Tung Sol KT 120s or Gold lion 88s in there and you will see what an amp it is
 the meek KT 90s were 7 plus years ago mostly sucked.
 That amp has great transformers.
 Best JohnnyR
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.
The GE ICs are very detailed and revealing so chose your tubes wisely. Your small signal tubes should not have extended highs.
It looks like the 12AX7s are in the gain stage and will have the most influence on sonics. Mullard CV4004 or the longplate square getter should be on your list to roll, they’ll take the edge off. Brimar CV4004 is also a good choice, more dynamic than the Mullard.
Then try a neutral 12AU7 which looks to be the driver/phase inverter.

As others have recommended, change out the KT90s. Don Sachs uses KT88s, a good choice. The GL KT77s are a wonderful linear variant of the EL34.

I wouldn’t recommend going on the merry go round of swapping cables, stay with tube rolling.
What brand of input tubes do have on hand?

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.  
 You would also have nice surprise you can update to Magnum II status
  Rogue Audio is in PA and if you are anywhere near they can bring that amp up to a beast level for very reasonable.
 The Atlas Mag II driving Vandersteen 3A Sigs Lou Reed and his fender on Animal Serenade live infuses a major smile and wow on everyone's face when listening.
      Best 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. 
I had Rogue Apollo monoblocks from 2010 that shipped with an all Electro-Harmonix tube complement - KT90, 12AX7 (gold pins), and 12AU7 (gold pins). That was indeed a very bright sounding complement. Almost ANY other tube choices you can make will be notably less bright; you don’t even necessarily have to go into NOS (though a few NOS tubes can be very bright, too). Start with Tung-Sol KT120 in place of the EL90. If you have Electro-Harmonix small tube, get rid of those too - e.g. maybe Mullard and/or Gold Lion reissues? I’ve also tried the Mullard EL34 reissues in a different amp, and liked those too. And a NOS short-plate Mullard 12AX7 will REALLY warm things up - in fact, too much IMO.