That being said, power output of an amp is governed by the OT primary load impedance, the power supply voltage across this load impedance, and the amount of current the power transformer can source. The tubes just control this current.
The whole "KT88/6550s have double the power" is something that got misconstrued by the fact that they can dissipate more power. Given the correct operating environment and surrounding circuitry, you can easily design an amp that will get double the power out of 1/2 the tubes (100 watts per pair). But the power supply has to be designed for it and the OT has to provide the proper load for it.
To summarize, if your power transformer cannot source the current to make double the power at its rated voltage output, you're not going to get double the output power no matter what tubes you put in your amp.