Thanks for your feedback on the yellowjackets.  I read an interesting comparison of the groove tube set up vs the THD yellowjackets (in an old black face bassman).  The groove tube power reducers did a much better job of both reducing volume and maintaining tone.  So, if no damage will be done, I might just try it.

As for the 6V6s, wouldn't you have to rebias the amp when you put them in?

I've got a 50w HRM, wired up by Nik.  Sounds wonderful, but I would like to be able to crank it up in small clubs to take advantage of power tube distortion.  As good as the amp sounds at low volumes with the preamp overdrive, it sounds much better cranked.  At 50 watts, it is just too loud.  Will the groove tube 928 power reducers, which substitute EL-84s for the 6L6s and claim to turn the amp into a class A amp, work in the HRM models?  Or, will the cause damage to the circuit and/or transformers?