Which one is that? The one adjacent to the RCA input sockets?
Yes, that is the input stage. The best sounding and quietist ECC83 you can find belongs there. To my ears I’ve not found anything I prefer to good vintage Mullards in the Stereo 20. It is by nature a bright and tight amp, very different to say a pair of Quad IIs. The best Mullards bring a beautiful natural timbre and open the soundstage up in a wonderfully 3d way without impacting the lively and fun nature of the amp. I’ve run mine with a full set of ‘60s Mullards and it sounds stunning, unquestionably real high-end. Day to day I compromise and run Russian 6P14P-ER power valves rather than wear the Mullard EL84s, and they are different rather than better/worse (they are brighter and more forward which can translate to more exciting on rock/electronica but maybe a bit much on some classical strings recordings). I run vintage Mullards everywhere else though, but if I had to have just one it would be the first ECC83. I’d happily pay £100 for a really good one as they just last and last.
I don’t think I have ever worn out a ‘50s or ‘60s Mullard ECC83. All the ones I’ve scavenged out of used vintage kit have all tested and work fine. Every last one. To the extent it doesn’t concern me that my Verdier preamp is stuffed with a date matched trio of 1960 Blackburn I61 type of Mullard ECC83 and by the nature of that pre are still ‘on’ whilst I’m playing CDs!
Sovteks, EI, JJ etc all wear, and some pretty fast. I’ve had actual fails of all of these, and every single EI went noisy within a few months. Every Mullard is fine, as are most US military (RCA, GE, Sylvania etc) 12AX7s too. I’d personally stick to Mullards in the Leak though, the US ‘tubes’ are often absolutely superb, but for me a bit too bright for a Stereo 20 or TL12 Plus unless you have very warm and laid-back speakers. Great in EAR, Prima Luna, Audio Research etc though.
PS I’ve never blown a Mullard GZ34 either. One of the main reasons I run them is reliability. Again they just last and last. I’m still too cowardly to sell any of my stash (I think I have six of them!).