Right channel distortion... Where is the issue?

Hi all,

I'm quite new to this world and I would appreciate some help as I troubleshoot a weird issue.

Brief summary of the issue: when playing some vinyl records, the right channel sounds distorted, especially the vocal tracks. The same vinyl does not sound distorted on the system of a friend who offered to try it.

My setup: Garrard 401 > Sansui AU-217 integrated amp > Mission 751 speakers. The amp and speakers were bought 2nd hand, I haven't recapped and/or cleaned anything.

Troubleshooting steps I already took:

- Clean the stylus with a brush

- Reduce tracking force

- Try other records: some records don't really display the same behavior, the right speaker still sounds a bit less rich than the left speaker, but there's no distortion happening. I needed to make sure that it wasn't about the record itself, so, as stated above, I gave it to a friend of mine and he managed to play it without distortion.

- Next step was to understand if the issue could be with the speakers themselves, or the amp. I tried to plug a digital source into an alternative amp provided by a friend, and there was no distortion with the speakers. Then I tried to plug the same digital source into my Sansui, and got a beautiful sound. I tried it with different slots (aux, tuner, even phono itself) and never experienced distortion.

- So now I'm back thinking about the turntable itself, but honestly I have no idea how to troubleshoot there. Also something puzzling is that the distortion seems to increase when I touch the source knob on the amp (for example, switching from phono to phono subsonic, which just seems to be a low filter). But this is probably because I haven't cleaned the amp itself.

Any suggestion for the next step?