6N8S 6SN7 6Н8С tubes?

They are both soviet tubes, similar but some have larger glass envelopes.

NEVZ means Novosibirsk Electron Vacuum Plant

If ever in doubt you can always compare the filament voltage and amperage by googling the specs of each tube. It’s a good idea not to ever exceed the combined amperage to prevent damaging other components in your equipment.

I am no tube expert but is has been my past experience that tubes with larger glass envelopes generally seem to add to the 3D experience and sound more open and holographic!

Oops!  I cannot edit my PO where I typed 6SN& meaning to type 6SN7 . . . I am a horrible typist.

Thanks, @ellajeanelle  , I will try to look that up to see how those two compare with the 6SN7.

. . . @ellajeanelle  , I just did a bit more surfing on ebay and there are actually similar listings . . .; they are quite affordable--I assume that means that they are not highly sought after and probably not all that great?

I am no tube expert but is has been my past experience that tubes with larger glass envelopes generally seem to add to the 3D experience and sound more open and holographic!

 As far as price goes, I haven't noticed much difference.  Sometimes the cheaper ones have sounded better to me.  I guess it's a matter of individual taste. Doesn't hurt to try them though.  I currently only have 2 preamps and a pwr. amp that are tube and those are in a room that I hardly ever set foot into.  I've moved more toward solid state.  The only amps that I'll always continue using that are tube are my Marshall guitar amps.

Best of luck!