I modded my Tremoverb when I found out just how cold the tubes run in the Spongy modes with the lower voltages - they're barely on (under 5W at idle in Spongy/Tube Rectifier), and it helps explain why I never liked the Spongy modes. I can see why it's like that because in Bold/Diode mode it gets pretty hot (at least by Mesa standards!) but it really needs some compensation when it's running with something like 100V less on the plates.
What I did was to use the stock Slave Level pot - removing the resistor and connections to it and the jack apart from the ground, since I never use the Slave function, and it could be derived from one of the speaker jacks with an external pad box anyway, should I ever need to. The pot is 10K, so I simply replaced the 22K resistor with a 12K soldered to the board in the same place at one end, and a wire to the 10K pot in series with it. So now the coldest setting is the factory bias (easy to return it to if I want, just dime the pot), and it goes usefully hotter but not over the top - the maximum on Bold/Diode is around 50mA, which is a bit too hot but not instant tube-melt territory - and I rarely use that anyway, I prefer Bold/Tube, which maxes at somewhere in the high 30s. So I can just dial it by ear in use and not worry about it.
But just in case I do want to know, I fitted a 1-ohm resistor in the cathode connection of one of the tubes (not the one immediately behind the jack since the reverb transformer ground connection goes there, but the next one), and hooked up the tube end to the Slave jack - so now all I have to do is plug a test lead with a 1/4" plug on one end and meter plugs on the other into the jack, and I can read the current directly - OK, only on that one tube, but if I'm using a matched set it doesn't really matter.
I would have to agree that making it variable (within limits) really is a big improvement on this amp and opens up tones it just wasn't capable of before, especially in the Spongy modes. Sorry Randall!