Max wattage for a JTM 45 style amp?

short answer is no. Some of the 50 and 100 watt 4 holers had PT's that could produce up to and over 600 VDC on the power tube plates. These amps were the ones that produced over the rated watts. A JTM45 has a PT that only spits out 180mA, and an OT that puts out 30-35 watts. Even with SS rectification and EL34's you may squeak out 40-45 watts tops

 

I've read that in a Marshall reference book - 50 watts at onset of clipping, 90 watts at a higher level of distortion.

Don't remember exactly, but my impression was that this wasn't for the older models, but rather the later JCM era models.

Not sure about the JTM45.

 

The output transformer impedance is key to getting the most power from an amp.
A properly designed JTM45 has a 6.6k primary which at the plate voltage available,is lucky to put out 35 watts at full song.
A JCM800 can put out about 65 watts if the plate voltage is about 475v.There are other factors as well like current and losses,but you get the idea.Distortion ideed does wierd things to the output.