It is good that could sort the treble out by changing the gain, interesting. My personal favourite setting was a gain of 5, I thought it sounded better than 1 to 4 and 10 didn't make that much of a difference to me, but I cannot recall what the details were why I preferred 5
. I should check that out again.
The sound signature you are describing points towards a preference towards Tung-Sol 6AJ5 (colder than M8100 and sweeter treble) only the bass will be lesser in quantity, although the 6AJ5-bass quality is good. In my experience you often trade better treble extension and more of it for a loss in bass quantity with these tubes. The CV4015 I am using at the moment is a "colder" brighter tube with very resolved treble and great in terms of focus and instrument separation. Then again you lose some bass over the Mullards, but it is of good quality. Actually JAN 5654W tubes could be a good suggestion. Clean and nice treble extension, not too much deep bass, but good mid bass punch.
I won't suggest the other from me preferred Mullard M8100 (with label) because I am not sure you might get the same treble issue you just experienced.
If you are interested in the JAN 5654W tubes, I could send them for a good price from UK; bought out of excitement a couple too many
Just on a side note, you are using HD650 headphones?
The risk about tube rolling is also the fun of it and there may always be some tubes that won't be in the selection later. Look at it as a little adventure
Tubes are not that expensive after all, plus over time you nail nicely down what you actually really like.