Sovtek 6N23P-EV / 6H23P-EB NOS for AI Modulus 3A?

In addition to my previous question: there is a hiss in one of my phono channels, which becomes louder when I turn up the volume. When I switch the tubes from left to right the hiss stays in the same channel. When I switch tubes from phono (6H23P-EB NOS) to line (new Electro harmonix EH) there is a loud and constant hiss in both phono and line (cd) but not influenced by volume control.
Is this a tube problem or something else?

Thanks

Yes, thanks, I have thought about that too. It is just that I live in Europe, which means when I order tubes from the US an extra 25-30 percent is added to the price + shipping costs due to VAT and customs.

Yes and thanks this helps indeed. I understand your reasoning that it looks like an electrical problem regarding the fact that the hiss stays in the right channel even when I switch the tubes from left to right and vv. But what I don't understand is why there is a hiss in both channels when I switch both tubes from phono to line and vv. Doesn't that look like a tube problem? And what could the electrical problem be? Any idea about that?

Re the tubes, I like clarity too and my system is not too bright at all, so I will go for the NOS tubes, even when it only is to replace the Electro Harmonics ones

If you are right then it would mean that the tubes of the phono stage need to be replaced anyway. But it could be a cap too. Difficulty is that as far as I know AI has no distribution network in Europe anymore

Thanks for your advice. Much appreciated