I think the difference between the 306 and your copper amp is negligible, but as you know I'm far from an expert in these matters.
I seem to remember I made a pretty significant jump on the 405 (possibly .5V to 1.5V), but that's just a guess and also with no knowledge of the gain on my Croft which is an old model I bought second hand. Before I made the change to the 405, the volume pot on the Croft was all or nothing within ~1/8 of a turn, but I remember that being a common complaint of the new edition Micro 25 (3-4 years ago), didn't Ken (Qwinn) commission a zero gain 25 for this reason? It wasn't just that it made the volume pot useable though, everything sounded so much better I remember saying to you at the time that it was like have a new amp, the difference was that significant.
From all the input from folk far more knowledgeable than me, I think my issue was local and you'll be fine.