The pot is accessed via flat head screw driver from the outside of the chassis between the power tubes.

As ususal, people seem to have definite but varied experiences that are at odds.

 Here's mine: I am player, not a hobbyist, but  I believe in doing basic maintenance on my gear. I read the articles on biasing and adjusted my amps accordingly. 42mA is right at the range of a "hotter'" bias, but within the "70%" recommended. I think you could get up to 45mA or so depending on your plate voltage which is the other part of the equation.

Now the counter: I have a great amp tech who does all the serious work on my amps. He insists that a lower bias is better and it does not effect the tone (I know, I know, the argument will start now). He says the tubes will last longer. I have done the bias comparison on my Bassman and Super Reverb and I have to admit, that he seems to be right. Since both these amps run 6L6 tubes and plate voltages 450-470 which is similar to the Overtone I think the same settings will work. My amp tech sets them up at 25-30 mA (brrrr cold). They sound great to me.

Matched tubes (+- 5mA, preferably less) seem to be much more important than bias, especially at higher volume.

Try lower the bias and let us know what you think

When I replaced the transformers in my super reverb, I had it set up for EL34s. I learned that the circuit has a lot more to do with the tone than the output tube. Ialso found out that I liked 6L6s a little better, but we better start a new thread if we get going about tube choices.