Another audiophile and MC275 owner helped me research and test different tubes:
Reissued Genalex Gold Lions KT88 for additional power or Genalex GEC KT88. 12AX7 / ECC83 / 7025 / CV4004 like Mullard and Telefunken or Tung-Sol.
I know the heat will be a problem for me as I transition to Florida.
lapierre