tube upgrade for first sound


Has anyone been able to improve on the stock tubes in the First Sound preamps? I was thinking about trying the Russian 6H23. Any thoughts? thanks
84audio
The only upgrade I have tried is the Siemens Gold Pin 7308, which
sounds superb to me. It's what Emmanuel Go of First Sound
recommended to me and is definitely one to try if you can find it. But
they are hard to find and expensive, so I'd be very curious to know of
good alternatives.
Try the Siemens CCa Gold Pin 7308 (as mentioned above) along with the Sovtek 0A2 as the other tube. Have them both Cryo'd, or purchase them already cryo'd.
The sound and dynamics will be improved.
I've very much enjoyed the Russian 6H23s I've tried from the Cable Company in my Audio Research SP9 Mk III preamp. They stomp the stock Sovteks into the ground. Worth a try.