The loose "rules of thumb":
- Upstream / input tube slots (e.g. "V1"), and slots that contribute gain (as opposed to follower or buffer tubes) are more sensitive to tube rolling, and you should use your best tubes here. But then there are also mu followers, which do both. You also want to be careful to choose very quiet tubes for V1.
- The older 1940’s VT-231 and early-1950s 6SN7GT tubes are generally sweeter, "airier", and better sounding than later GTA and GTB variants. This is where the 6SN7 gets its beloved reputation ("king of tone", etc). Unfortunately, they also tend to be noisier. It’s hard to find a quiet Ken-Rad or Tung-Sol black glass VT231, no matter how sweet they may sound. Microphonics are particularly a big issue here. So it’s a catch-22. You want to use the sweetest tubes in your V1, but then noise issues might make this a poor fit. I’ve generally been able to use old 6SN7 tubes in V1 of power amps, but NOT in preamps, due to noise. If you have to find a quieter tube for V1, you can try early-make GTA or GTB tubes (1950s better than 60s). They are a LOT quieter. I like the Sylvania tall-bottle "chrome dome" 6SN7GTA, but still prefer older 6SN7 tubes. Or you can try modern Russian / Chinese tubes.
- 6SN7 aficionados LOVE to match 2 different makes (where possible), rather than stocking all slots with the same make (e.g. Tung-Sol round plates in V1, Sylvania VT-231 in V2). With 6SN7 tubes in particular, this seems to sum up the positive sonic attributes of each make with little downside.
- The modern Russian EH 6SN7 and Tung-Sol GTB reissues are solid tubes. They’re not the most refined or sweetest sounding 6SN7, but they do a good job, and their transconductance is going to be 20-25% higher than any vintage tubes you find - VERY strong. I believe these tubes are a natural fit for output and driver slots (e.g. driving KT88 or KT120 power tubes). They have a clean, punchy sound - "rock n’ roll". They can also be selected for low noise & microphony. I don’t love them in V1, but if you have too much noise with the older types then you can try them here.
- Everyone who’s been in the 6SN7 game will develop favorites. There are some patterns and general agreements on what’s better or worse, but nothing’s written in stone. I really like the Tung-Sol Black Glass Round plate VT-231 - others will say they’re way overrated! Some really like the NOS Russian "Melz" tube, but I haven’t heard them myself (said to be the Telefunken of 6SN7, if Tele made this tube).