this is an interesting issue. I am very alert to distortion and will study it until i figure it out. (OCD for me). I have found that the first culprit usually is gunk on the cartridge which you indicated you checked. If you have a good magnifying glass i would take a good look at the diamond to make sure its clean and not damaged. I have seen a small amount of gunk on the stylus cause the issue. Magic eraser usually good for it. I have seen a worn/damaged stylus will cause some distortion that is hard to troubleshoot.
You have seemed to cover most of the items that typically make sense, there is an issue with resonance that i have heard can cause this type of issue though i have never experienced it.
The gain/loading can affect the clarity but i find that is not record specific but more of tone related (ie cymbals or the highs get tinny, etc) and happens on all records that have similar tones
Let us know what Soundsmith says. I have been using ZYX a long time and have found them to be excellent trackers once zeroed in on vtf and vta
You have seemed to cover most of the items that typically make sense, there is an issue with resonance that i have heard can cause this type of issue though i have never experienced it.
The gain/loading can affect the clarity but i find that is not record specific but more of tone related (ie cymbals or the highs get tinny, etc) and happens on all records that have similar tones
Let us know what Soundsmith says. I have been using ZYX a long time and have found them to be excellent trackers once zeroed in on vtf and vta