6692: what is the best upgrade to aim for?

I have ARC PH3 hybrid phono preamp that I want to upgrade in the future.
Currently I use Sovtek 6692.
They are 6922's in the PH3, and I use 3 Siemens 7308 gold pins that I bought from Upscale Audio. They sound great! There are millions of other possibilities as well, including Mullards, Telefunkens, and Amperexes.
I just purchased some 6H23 Russian tubes from Conus Audio @ $15 each for my REF-1. Big improvement over the stock 6922.
Yeah the 6H23s sound better than the stock Sovteks in my BAT VK3i pre. Not as good as the Siemens E88CCs, but I have been having some microphonics problems with the Siemens. The 6H23s from Conus are way cheaper, too.
Some advice please.

If I contact conus or upscale, are they trustworthy to be able to recommend good upgrade tubes for my BAT vk-3i? I am not knowledgeable about tubes, but would like to experiment a bit.