Distortion at Peak Dynamic Levels

I generally second ZD542's post. The most likely culprit is either (1) phono stage overload/slewing distortion, or (2) mistracking by the stylus.

If the problem is the latter, you may be permanently damaging your records. So you should investigate that first.

After making sure your records and stylus are really clean, try playing an offending passage whilst increasing VTF in tiny increments (like .01 or .02g if you can). Make several trials, increasing VTF each time. Does the distortion lessen as VTF goes up? If so, mistracking is the culprit. Eliminate it immediately by improving your setup, else you risk destroying your vinyl.

If raising VTF several times makes no improvement, the odds are it's your phono stage. Frankly, that's my bet. Many phono stages are incapable of reproducing powerful vocals or hard blown horns (for example) without slewing distortion. This can sound exactly like mistracking to all but expert ears. This can happen even if even if the phono stage is perfectly gain-matched to the cartridge. You should check that, as ZD542 said, but you may just need a higher quality phono stage.

A good VTF scale is indeed essential, but why spend money on a test record? The vocal tracks he's having trouble with provide exactly the test tracks he needs.

If he gets those playing cleanly, all will be well. Artificial test tones won't make it any easier. Nor will they provide any more useable information.

Like you I have multiple test records, including Shure, HFN&RR and others. None of them has been out of its sleeve in years. I've set up dozens of rigs from entry level to very high end without needing any of them.

Hmmm... it would make more sense if your downforce had been too low, as that can allow mistracking during challenging passages.

Still, some of the factors Tonywinsc mentioned may be involved. Or perhaps the excessive downforce took the coils so far out of alignment vis-a-vis the cantilever that certain frequencies were being distorted.

I'm guessin' wildly here! In the end, if it sounds good then it probably is. Enjoy the music.