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

Before I respond to your post assuming it is a tube issue could you clarify for me what you are hearing.

As described I interpret your posts to say that when your original tubes (the 6H23's) are in the phono stage you are getting a hiss in the left channel. This is a problem typically associated with tube noise level (quality).

But you then say that when you change the tubes in the phono stage from the left channel to the right channel the hiss does not go away. That might mean that you have an electrical problem in the left channel alone! Otherwise if the tubes in the right channel were quiet in the right channel and became noisy in the left channel and conversely the tubes in the left channel were noisy in the left channel but became quiet in the right channel then I can only conclude it is not the tubes. This is furthered by the fact that when you put these same tubes in the line stage both channels became noisy!

I think you need to have your pre-amp visited by a technician.

FWIW, re tubes, new production EH6922's/6H23's (same tube I believe despite designation) and the NOS 6h23's, there is a substantial difference in tone if nothing else. If clarity is important to you, as it is me, I think the 6h23's are much better and very similar to NOS Sovtec 6922's if not identical if the noise level is the same. The EH tube is muddy in comparison. However if you think your pre-amp is already inherently bright and needs to be toned down, the EH's would probably do the job and I believe both are rugged enough for your pre-amp.

Hope that helps a bit.

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.