Here are some listening notes from experimenting with a variety of 6SN7 tubes in my Cary SLI-80 integrated amp (used as a driver).
Tungsol GTB re-issue
These tubes are really quite good. After chalking some hours under their belt, these tubes have good handling on both bass and treble extension and have a smooth and liquid midrange. I find the Tungsol to be a wee bit less transparent than the EH, but the Tungsol has the heft and liquidity that could be said to be more musical. Dynamics in particular are superior on the Tungsol. Instruments and voices also tend to have more tonal density and bigger image size compared to the EH.
Electroharmonix
Visually, the internals of the EH tube are almost identical to the Tungsol re-issue. The plates are of a slightly different colour, and the halo getter is larger and the bottom part of the ring is attached to the support rods, compared to the Tungsol where the top part of the ring is attached to the support rods.
Sound wise, both tubes have quite a lot in common but also enough differences to justify choosing one over the other.
Overall, the EH is a little bit lightweight in comparison to the Tungsol. The Tungsol has a firmer and meatier bass and a thicker midrange. The EH has a good shimmery top end. Instruments are well separated but are a bit lacking in height and depth.
To sum it up, the Tungsol has a more romantic balance, while the EH has a more modern and analytical feel to it.
General Electric GTA D getter
This tube has the strongest feeling of “being there”. The sense of acoustic space and the 3D soundstage projected are its strong points. Its weakest point is an overall lack of bass weight, and an energetic top end that seems to emphasize sibilance and get lost in complex mixes. Its strong points warrant it serious consideration but in the end I preferred the more even handed presentation from the RCA and Tungsol.
RCA GTB Black Plate
The RCAs project good acoustic space and strike me as being a better behaved version of the GE. Its bass is not as impactful as the Tungsol but has better weight and extension compared to the GE. It has a denser and creamier midrange than the GE and more organized and controlled highs. It is not quite as holographic or transparent as the GE but is more balanced in its approach.
Cary labeled 6SN7
These tubes came as stock in my Cary. I am not too sure who manufactured them but looking at their construction, they look quite similar to Shuguang tubes.
This is a very soft and gentle sounding tube, with a rounded bass that tends to diminish the impact and dynamics of the music. It has a very smooth midrange that is thick and creamy. It is not really a bad sounding tube, but tends to convey music in a warm and syrupy kind of way. It has a reasonably open top end that is well behaved. Well suited for taming a bright system, but it’s a bit too heavy handed for my taste.
Conclusion
My two favourites of this test group are the RCA Black Plates and the Tungsol Re-issue. The Tungsol in particular is affordable, widely available and a decent alternative to NOS / used vintage tubes that are becoming increasingly difficult to obtain cheaply, especially in Asia. The Electro Harmonix should not be written off and should be considered if a leaner and brighter presentation is preferred.