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

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

The reason for the hiss in both channels in the line stage is likely because the tubes, both of them, are just too noisy for the lines stage. This is counter intuitive because typically phono stages require quieter tubes than line stages, i.e. tubes that will work in the phono stage will work anywhere and tubes that will work in the line stage will not necessarily work in the phono stage. In my pre-amp I use NOS 6H23's in the line stage and a combo of 6H23's and Sovteks in the phono stage but I'm not using an AI.

Re cause of noise in the left channel of the phono stage - a guess or course, but it might be a cap failing?

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

Perhaps a local technician could diagnose your problem and fix it. You don't always need the manufacturer to repair thier products, although some times it does help if you can do so. Check around and find a recommended repair agency in your country, one that has experience working with tube products. Call and ask what they can do if not local.

Thanks for your advice. Much appreciated