tube upgrade for first sound

I have some cryoed Telefunkens in my First Sound. The are fantastic - dead quiet and dynamic. Beautiful sounding tubes. I bought them from TubeWorld.com. They are costly but I really wanted the return policy if I did not find the performance justifying the price.

thanks for the suggestions. I have tried both the Russian NOS 6H23s and the Siemens gold pin 7308s since starting this post. The 6H23s are a nice improvement on the stock tubes. They are smoother and have a bigger soundstage, and I think they are a good value, but the Siemens took me to a new level! They are unbelieveably fast and transparent, much more low level detail, better width, depth, better placement from front to back, more natural voices----you get the idea. Unfortunately, they were obscenely expensive, but worth every penny to me. Now I understand why Emmanuel Go recommends them for his preamps.

Amperex 7308s (USN-CEP, or JANs, or PQs).

AMPEREX PQ A-FRAMES, HOLLAND MADE MULLARDS & TUNGSRAM WERE THE BEST I'VE USED. THE SIEMENS WERE TOO MUCH LIKE SOLID STATE.
THE AMPEREX & MULLARDS LET SECTIONS OF THE ORCHESTRA LIFT OUT OF THE ACOUSTIC ON CRESCENDOES JUST AS IN LIVE PERFORMANCES.
THE ABSOLUTELY BEST MIDRANGE WAS FROM EARLY PRODUCTION (50'S) BUGLE BOYS, BUT THERE LIMITED DYNAMICS/EXTENSION WAS APPARENT ON ORCHESTRAL RECORDINGS, BUT ON SMALL SCALE RECORDINGS, THEY ARE THE BEST.
FOR NON-CRITICAL LISTENING, THE ROCKET LABEL 6H23'S FROM UPSCALE AUDIO WERE A GOOD CHOICE FOR ME.

CORDIALLY,
RAMSES