A little lean/timbre was off at times emphasis in upper mids may contribute to this. Sonorous, well-extended bass, but lacking some weight at times (decreased upper bass, and a little unbalanced emphasis from higher frequencies may have contributed) The slightest hint of graininess can sometimes be detected on a few notes. Does NOT suffer overtly from treble grain problem, but can be a little fatiguing on very high notes not as smooth as a good treble could be. Very detailed across all frequencies, effortlessly airy and spacious presentation. I presume this is due to the tube not being as linear as it could be. It is not as obvious with my HD650s, but most 12AU7s, bar the best of the best, exhibit this problem- sometimes painfully so- through the HD800. I have tried to summarised each tube's characteristics in 3 words as well as some listening notes as I went along, both for my own quick reference in future and for anyone who wants to get a brief idea of their differencesДЇor reference, I find that the 12AU7 family is a little grainy and harsh, especially in the top registers. The listening was conducted with a 5998 to maximise the input tube's abilities, and HD800 to hear every detail available. I then compared them to a pair of Mullard 6J5GT tubes. Their market price is above and beyond what I'd be willing to pay for these mini guys, so I am super thankful I get to test them out! Other tubes were the Telefunken smooth plate ECC82, Hivac smooth welded plate CV491, and a regular Mullard Blackburn Gf1 (1963) ECC82. I was recently loaned an Amperex 7316, generously by a fellow BHC owner on another forum. Inspired by some recent posts here and on the 6J5 thread, I decided to do a mini shoot-out of the 'top' 12AU7/ECC82s.

Bottlehead crack w/speedball