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.