Rogue Atlas Magnum sounds bright -- any recommendations?

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.

bdp24
George, I've never heard (or even seen) a plasma tweeter

These are what I use with the ML Monolith esl's but there are a few new variation today.

https://www.ebay.com/itm/Magnat-Plasma-MP-02-ion-tweeter-massless-air-plasma-speaker-Plasmahochtoner...

https://www.lansche-audio.com/products/plasmatweeter/

Cheers George

Try changing the speaker taps and see if that helps!