Electro-Harmonix can be a little brittle at the top. Have you given them at least 20 hours? I've not heard the 90s but I have EH KT88s with that attribute. You may do better with the Tung Sol KT120. Also consider rolling the small signal tubes.
Kt90 Upper midrage forwarness
Hello,
Running Rogue Apollo amps with EH KT90's
Im getting an Upper mid range that's out of balance and Forward.
I have had other rogue amps with Kt88's without the Forwardness.
Considering going to EH kt-88's or Gold lion Kt88's
Have High efficiency speaker which is adding to this.