Safebelayer, Just to be sure you got the right idea from Almarg, me, and a few others, what we suggest is that one channel is 180 degrees out of phase with the other. That condition can cause most of the symptoms you report.
So, to check that problem, just reverse the speaker leads on ONE of the two channels, either one, leaving the other channel as is. Perhaps you know this and have already tried it with no success, but it did occur to me that perhaps it wasn't clear.
For sure, no cartridge sounds its absolute best on day one, but not to the extent and with the peculiar symptoms that you report.
So, to check that problem, just reverse the speaker leads on ONE of the two channels, either one, leaving the other channel as is. Perhaps you know this and have already tried it with no success, but it did occur to me that perhaps it wasn't clear.
For sure, no cartridge sounds its absolute best on day one, but not to the extent and with the peculiar symptoms that you report.