Stavros is a regular tube bloodhound.
I cannot help but think that, amongst the thousands of tube types made, there has to be something out there nobody has really tried out for audio use that has good curves/specs and is still really cheap. Tube rolling the 3DG4 was a genuine delight at $1 per tube. Truly guilt free rolling there, and such a thing could be a really strong selling point for an amp design. You can experiment if that is your thing and not have to spend a fortune on the same tubes everybody is driving the prices up on all day long.