Not sure if these have ever been discussed here, but decent budget tube alert! I read on some web page I can't even remember, that there were some cheapo 68NP tubes, a direct equivalent (or so I was told) to 6SN7 coming out of China. The thread was very divided on the subject - some people reckoned they were amazing, others were a bit sniffy.
I got a pair, just to see what they were like, since they were only £17 for the pair. They arrived very quickly and I gave them a whizz with Chatham 6AS7G as powers. This was when I was using my Quad power amp, which I've come to realise, has very specific tube combos which work, but not much else sounds any good. The 68NP sounded pretty flat, not terribly good in any department. I let them run a while, gave them a chance, but they didn't shape up - if anything they seemed to deteriorate as they burned in. They certainly looked pretty tatty too.
However, I just randomly tried them again, since I'm revisiting all my tubes to reassess them in the light of getting my Bantam Gold amp. What a difference. In this setup, with Mullard 6080, they seem to be a bit of a decent all-rounder. Their special talent seems to be highlighting the frequencies which accentuate reverb. Orchestral stuff sounds really 'live' and involving. But they're certainly a good achiever in the mids and bass. There's a real absence of harshness to my ears.
Those observations were from use of the Elise as a preamp, over speakers. I just tried a comparison with the HE400 and they still do quite well. I'd say with those headphones there seems to be a very slight hole somewhere in the mids, which I can't quite put my finger on yet. Will have to give them more time later on.
Anyway, I'm a big believer that any tube's performance are at least 50% dependent on the other components in the chain. But for £17 (and you could probably find them cheaper) they're worth a stab. I'm certainly enjoying them a lot right now. A lot of depth and involvement.