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

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