The NOS Sylvania GTA in my deHaviland UltraVerve sounds very good. Good luck.
I have not tried the Electo Harmonix, though, I have not heard good things about them.
I've tried KenRad VT231's, RCA red-base, and CBS-Hytrons. Each sound quite different, and I'd recommend trying several of these versions to discover the flavor of sound you enjoy the most. Either way, they will be a huge improvement over the stock tubes (I only use the stock tubes to burn in interconnects).
Also, if you have the DJH version, you'll want to roll the rectifier tube. EZ81's are the way to go.
I realize this is not a lot of help, but it's the most honest response; there is no 'best' tube for this preamp. Instead, it's a matter of what your personal tastes are, and what tube is the best vehicle to deliver that sound.
I am currently using a pair of CBS Hytrons which sound nice. I hav eused Sylvanias and Tung Sols with great success. I have also found that I can achieve some nice improvements by changing the 6CA4. I am currently using a Mullard Branded as an Amperex EZ81 in place of the stock tube. I have never heard the EH, prefering to use NOS whenever possible over anything from EH. Moral to the story is experiment to tailor the sound to your musical tastes.
KenRad deserves their reputation for being bass rich. They also deserve their reputation as being microphonic.
RCA is sweet & airy.
CBS Hytron is quick and uncolored (for a tube).
The EZ81 I have is a Haltron.
FWIW, I plan on cycling through most of the NOS tubes out there. Still on my list are the various Sylvanias, Mullard, TungSol, etc.
Like it or not, with this preamp and revealing down stream equipment, you'll find the tube drives much of the sound you hear; so it makes sense to look at tube rolling as a long-term ride.
I purchased a AES/Cary DJH preamp along with there six-pacs about a month ago.(very impressive set) I also felt I needed to change out the stock 6sn7 tubes in the pre/amp, I did buy the Electro Harmonix with the gold pins, and YES there was a definite improvement in sound, The sound become smoother more defined, detailed and in a way cleaner, not that the stock unit was bad at all.
I think it was the best $ 53 I spent in a long time, and I want to replace the six-pac with them when I find them .
I upgraded my 6sn7 tubes with EH tubes because the cost is reasonable. They tightened the bass and extended the highs. My system has better drive and control and is more refined than with the stock tubes. There may be better tubes but at $12 each for the standard version you can't go wrong with this experimentation.