1. Make sure the Vibrato controls (speed and intensity (depth) )are all the way down, or if you have the footswitch, make sure the vibrato is off. Same with the Reverb control.
2. 100 mV is a little hot. you may be over driving the front end a little. Drop down to about 43 mV, which is closer to the output of an electric guitar.
3. Does the anomaly appear on both channels?
4. On both channels be sure that all three EQs are set in the middle, around 5.
There are a couple versions of the Twin Reverb. The original point to point wired ones were rated at 85 watts. Then there are the Master Volume models that were rated at 100 watts. Some of those models had what they called an "Ultra Linear" output, whatever that meant. It was designed to make the output as clean as possible. The newest models are a modern day replica of the original 85 watt model, but with a PCB instead of point to point wiring.
If you are measuring 70 watts out into a resistive load, the tubes are in good shape, and the owner is complaining of low volume, the trouble could be the speakers. Take it off of the load and put a different speaker on the output. Be careful as 70 watts, like that amp, can be stupid loud. The speakers could just be blown.
Good luck.
Rick