These 4 hole (4 input) type amp rely on each stage contributing a little for their distortion tone. When we think of "power stage" distortion, this includes the phase inverter which helps to fatten the tone up when overdriven. Fitting a PPIMV to a pre PI master volume type amp & doing a little experimenting will show this to be true. The power tubes, OT & speakers all contribute too. The thing is, as Cadorman noted earlier, they do get loud when pushed to overdrive. At full clip a 50 watter can put out close to 70 watts.
While cranking the volume & limiting drive to the output does result in preamp overdrive, it's just not the same, even when the PI is included. I've tried every type PPIMV, I just don't like them much for the same reason, it's just not the same.
When necessary I use a Weber Mass 200 attenuator to knock just a few db off (no NOS power tubes fitted for this), but prefer not to.
All that said, a JTM45 is a 30~35 watt amp, you may get away without needing to limit drive to the output. Cheers