So I went to see the shrink, and now the other me solved this problem - let me explane this the best I can
When my NAD broke down last summer, I dug up our old AudioTehnica (made in the USSR in the 80's) which is using 5-pin DIN connectors. So I went to a nearby cheap store to get the right cable. After some time I noticed the problem, and suspected the cable immediately. I got a new one, this time checked by the salesman. He said the connections show ok.
Today I was listening an older jazz-album, and noticed a strange sax echo from the left channel, which made me think of L-R-GND -configuration instead of L-GND-R. Measured the cable myself, and fixed the problem by re-connecting. Sounds great.
Some audiophile I am, really... but the case is now closed.
I am truly sorry if I caused unnecessary head-scratching to anyone.