My data sheets show the heater current for the 6L6 is 0.9 amps. For the KT66, heater current is 1.3 amps. Otherwise, they seem to be pin-for-pin compatible. Now, I'm not knowing the spec's on your power transformer, if it can handle the additional 400 mills of heater current. You would have to plug the tube in, and see how much it pulls the heater voltage down with it installed. I think anything below 6.0 volts is going to make the power transformer run pretty warm. Anything below 5.7 volts, and that will make the rest of the amplifier unusable. That low of a filament voltage, and it will just kill the tone coming out of the pre-amp.