Thanks to DecentLevi for pointing out the 6A6 and 6N7 tubes which I've had a chance to test out. Mercifully all tubes arrived intact and had good parameters.
Both tube types are similar with the same characteristics and also share a similar sound. The 6A6 predated the 6N7. They are characterised by a warm, euphonic, capacious, solid, easy flowing sound, reminiscent of floor speakers. The immediate most obvious difference from other octal dual triodes I've heard is that they are louder, amplification factor 35. This shows as a general increase in all frequencies, with no tendency to differential upper- mid brightness. The bass is very well presented, with the different layers easily differentiated from each other. Listening to Schubert's String Quintet it was easier than I have ever experienced to differentiate the two cellos, and viola, even though they are sitting very close together, not just by the position and imaging, but also by the different timbre of the instruments. The same applied to the same composer's Trout Quintet, where the double bass, which usually just sounds mixed in with the cellos had very clear individuality. The mids are quite forward, but not claustrophobically, the highs were slightly attenuated on all tubes except a 6N7 RCA and 6A6 Ken-Rad. The Ken-Rad was actually surprisingly bright but it settled down playing overnight, so it maybe an unused tube. In general I would say the treble on these tubes is realistic, without being emphasised.
The 6A6's had a bigger sound-stage than the 6N7's, I hesitate to suggest that's due to their large coke bottle shape-but it's interesting all the same. Their sound-stage is the largest I've experienced with Ember, the 6N7's are normal bottle shaped and share a similar sound-stage to FrankenEmber, with a smoother, warmer sound, no treble brightness, no muddy lower mids which some people (including myself) have occasionally observed with the otherwise excellent FrankenEmber.
Visseaux and some other 6J5 owners don't need to worry though-these tubes don't quite have their energy or fine detail, but with a brighter can they sound very impressive to me. As an example they suit the HD600 very well, more so than the HD650. They are quiet, just a mild hum with my X2. There was noise turning the volpot unless I invoked the input capacitors.
The adapters work fine though the 6A6 to 6SN7 is a behemoth monster. Ever helpful Mrs Xu Ling has now specially produced an ECC31 to ECC88 adapter to avoid using an additional 6SN7 to ECC88 adapter.
Tubes: 6A6- Sylvania and Ken Rad.
6N7- Brimar, RCA, both glass bottle, Ken-Rad, metal.
Sources: X5 (FLAC),CD6004, PDR-609, M-DAC, Bushmaster 2, NOS DAC's, Muse and Mini TDA1543
Music: Various Classical and Jazz.
Disclaimer: Just all my personal initial impressions of a tiny selection of these tubes.,
Here's a couple of articles with some info. Apologies if they have been posted before.
http://vinylsavor.blogspot.co.uk/2013/02/tube-of-month-6a6.html
http://vinylsavor.blogspot.co.uk/2011/02/tube-of-month-6n7.html
Hope to hear more people's experience with these tubes soon.
Thanks for taking the time to post this. It is a very nice review of these tube options, Howie13!