Rogue Atlas Magnum sounds bright -- any recommendations?

Call Rogue. They answer the phone and are extremely helpful.

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.

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.

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