6SN7GT VT231 TUBES

All of them are pretty great, and the final choice is going to be very system & listener dependent. More often than not, I like Tung-Sol black glass round plate 6SN7 the best - if I had to pick one, that's it. Unfortunately, I've had the bad luck of heaters going out on a few of these. 

I prefer Sylvania (side getter) VT231 over RCA. Sylvanias in general (through the 1950s) are pretty cool with me. Next are Hytrons and Ken-Rads. And RCA last (but still nice). I haven't had the pleasure of trying the Raytheon VT231, but they look lovely! Heck, even the Russian issues (Electro Harmonix, Tung-Sol) can be good - they measure VERY strong in transconductance, and can sometimes be best-in-slot for driving lots of big KT tubes. 

What I found is that vintage 6SN7GT tubes are best suited for the input stages of power amps. In these slots, you're getting all of their sweet sound with none of the downsides. In preamps, these old tubes are going to have all kinds of noise / microphony issues. Even worse in phono stages. As an output stage tube in headphone amps (this was more popular circa 2000s head-fi), they are very weak compared to many other better alternatives. 

For preamp slots, it's best to stick to later GTA / GTB issues (lower noise & microphony), or modern Russia/China tubes which can be selected for very low noise (and matched tightly, to boot). 

6SN7 is a GREAT tube, but it's important to respect its proper role and limitations. 

@vintageadict 

RCA, Sylvania, Tungsol 6f8g are good.

But Tungsol seems to be overpriced due to hype.

Raytheon sounds more shapr and clear thus dependent on system and taste.

Thomas

Interesting that I mainly use the 6SN7 tube family in my preamp and have not experienced any noise or microphony issues with older tubes.  Just a couple outright failures.
My preferred 6F8G tubes are Raytheon with flat ladder plates (very hard to find).  Step up from the T plate version.  National Union round plates and RCA Gray Glass are also excellent.

@shkong78      Are you still on the Melz 1578 bandwagon? I bought two pair from an ebay seller you had cited. My experience with them was dismal as none of them made 50 hours use. I just ditched the last pair a few days ago. Don't get me wrong...I'm not blaming you for my outcome, just wondering if you've had better luck with longevity.

I have 3 pairs of Melz but no more interest in them due to noise problem.

I also collected more than 8 ohter Russian tubes whish also sound decent.