I pretty well agree with Johnny on this. The PPIMV is a lot better at giving a cranked sound at lower volume than any other sort. It will not sound exactly the same at living room or even garage levels, but will be so much closer you'll be happy with it. There's no replacing the impact of that air movement or the electricity of 500V tubes at their extreme. But it completely not suck.
Even an attenuator doesn't get exactly the same sound if you use it for more than just taking the edge off.
I'll quibble with his cost and time estimates being optimistic.
Be aware that power tubes have a spec for the grid resistor that you are replacing with a pot. EL34 don't want much more than 100K there.
One thing I like to do with these is use a pot larger than the value I want and bypass it with a similar resistor from wiper to the bias line. Say a 500K Pot with a 470K resistor. Smooths out the taper and ensures that no matter how bad that pot gets it will never lose bias on that tube.