Problem with the MKIII I had is a faulty volume pot, I think. Read below to see the whole story and say if you think the volume pot is a problem too.
For a short version of the question... which volume pot should or can I buy? Should I just get the same Alps 16 100KAX2 as stock, or is it worth while to upgrade to a better pot?
Now, for the problem:
I bought a used MKIII. Worked fine for a few hours, then left channel stopped working completely. Right channel worked fine. White RCA input signal didn't reach the headphones, but input going trough the red RCA did work. So I tried replacing tubes, different power cable, different RCA cable, source, fuse, etc. Nothing changed, always right channel worked, left didn't.
So I opened up the amp and saw some cables fell off of the volume pot. While trying to get the volume pot off, I ripped off the rest of the cables. Now I soldered them again, in the same order as they were before and as they should be soldered, and now the LEFT channel works, but the RIGHT one doesn't. And its not the case of me soldering wrong channels, because now the WHITE rca input works, but the RED rca doesn't, so its the opposite to before. My guess is the reason for that is my poor soldering, because it really is poor.
Also, the volume pot sometimes seems smooth when turning, but sometimes its rough. Like there are spots that are rough, and not always on the same place, like there's something inside the volume pot moving around and screwing with the signal. I guess I'll just have to replace it.