Never worked on a DRD, but have scratch built a Loftin-White as well as OTL, SET & etc. If you look at the schematic on the E-P web site for the single 300B version (closest one to yours I see) it uses a Sovtek 300B. Biasing instuctions are on the schematic: first you adjust the 300B bias, then with the other adjustment, adjust the driver voltage. I assume you have a 2nd adjustment for driver? Since the amp is direct coupled, each adjusment will effect the other, so you'll have to repeat the adjustments a few times. If there's no adjustment for the driver tube, you'll likely have to do what I have to do with my L-W, ie. try several different driver tubes until you get one that gives the right voltage. You'll obviously need to flip the amp over to do this work. You'll need a voltmeter & a dummy load resistor or a cheap speaker to load the output. If you don't feel comfortable working around high voltages, by all means take the amps to a good tube amp repair person.
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