With the HD-650s the Mullards had certain glowing liquid qualities in the mids that none of the others quite had. But for lots of music it just turned into rich warm-soup. It was fun... & I like soup - but probably not all the time. It could sound too slow. Noticeably rolled at both ends but nice texture in the middle. Lovely with intimate, organic sounding music with limited number of real instruments and a very small dynamic range. More complex bass-lines could sound quite muddled. Lacking in precision & definition. Thick. Air, detail and separation is sacrificed in favour of a mid-centric experience. Luscious & smokey. Juicy. But Soft as hell. Compromises the 650s technical abilities. I quite like it though.
GEC 6AS7/A1834 - seemed to be a more complete & dynamic tube. Less warm but still presents a good dose of the classic sound. Sharper resolution, faster & a more even tonal balance from top to bottom. However, since they cost almost as much as the amp I am using, I think the value in my case is probably pretty questionable.
I feel they offer a slightly different experience to the GEC 6080. The 6AS7G seems to have a grander scale and solidity to the notes. It sounds very pure, natural & unfiltered to me. The GEC 6080 is perhaps marginally more restrained, delicate, but has some imaging prowess that might even be better than the big bottle. In comparing the two GECs, If someone wasn't wild about the 6080, I probably wouldn't recommend the 6AS7G - they are most definitely from the same stable. Probably safe to skip it.
For me - all the above NOS British tubes are on the warm side of the track, and I would choose something US (or Soviet) e.g. the 5998, Winged-C, Bendix, or Tung-Sol 6080 for a sound that is drier but with more impact & bite - if that’s your thing. These might also allow a greater impact & range of choices with driver tube / headphone changes too.
This is with Rega Dac / Stock-Crack / HD-650 / Mullard CV4003 as driver.
YMMV and all that jazz.
