Did you try a Mullard/Brimar valve?
Stock china valve.
Absolute junk - does improve to garbage after about 2hrs.
So warm it is unusable, mid & vocals aren't great either, sort of a nasty mucked up 50s radio.
NOS Brimar valve.
Shockingly better - out of the box, and should get better over next 2hrs.
Slightly warm, but not thick. Bass is deep, controlled. Vocals are airy, highs far better.
Adele's vocals on quieter tracks sound absolutely haunting.
However this is an amp that does not like extremely complicated music, musical fidelity headphone DAC amp (HPA) was better in that respect - BUT could become tiring quite quickly.
There are 2 Gain jumpers.
Stock is both set to HIGH not Low, so I may try setting them to Low at some point.
Might also nosey around the PSU caps as that is an easy. Not sure about this one, NOS Mullard/Brimar makes it auditionable, stock tube is a toilet.