Those are dark, dark, dark amps. JJ 6L6's are the ticket in the power section for those, but also be mindful of the bias. There's a sweet spot between cold and grainy, and too hot and mushy.
You REALLY have to crank the treble and presence on those because they are so dark. I went with a Tungsol in V1 in mine, and that did brighten it up. The older ones, (before China had issues), came stock full tilt with Ruby's, (all Shuguang). I didn't like their 6L6's. I always did a Tungsol in V1, JJ in V2, Ruby in V3, and Sovtek in the PI with JJ 6L6's biased to 34-36mA. Those are killer sleeper amps with the right tubes/bias. Leave the boost off, and hit it with an OD instead!