There is a probable cause. The 7027 tubes use 0.9 amps each at the filament (heater) where the KT88 uses 1.6 amps each. I do not have specifications on the PT but the filament windings are being taxed heavily maybe causing hum. There is (should be) a hum adjust control in the filament (heater) circuit. Make sure it is adjusted for least hum.