Good info. What you're hearing is called "inner groove distortion (IGD). It's a very common problem, particularly on strong, high pitched notes or massed violins.
Unfortunately, knowing the name does not tell us the cause. IGD has many possible causes including, but not limited to:
- cartridge or stylus incapable of accurately tracking the shorter wavelengths on inner grooves
- cartridge capable of tracking, but mistracking due to improper VTF or anti-skate
- cartridge electrically or mechanically incapable of resolving closely spaced, high frequency fundamentals and harmonics (I've heard many that can't)
- cartridge overhang and/or zenith mis-aligned
- LP damaged by previous plays
- LP not clean
- stylus not clean
- cartridge/tonearm combination resonates internally at certain related frequencies
- phono stage overload or slewing distortion (a very common cause)
If you hear crisp, static-like bursts or fuzz from one or both tweeters during this distortion it's likely your stylus is mistracking. (Listen with your ear on line with each tweeter.) If you hear this, STOP playing valuable LP's until you eliminate that sound. Mistracking means your super-sharp diamond stylus is literally bouncing off the much softer plastic groove walls several 1000 times/second. The usual cure is more VTF and/or (if the mistracking is in the R channel only) more antiskate.
If the sound is just distorted, without those static-like bursts then mistracking is probably not occurring. You'll have to look toward those other causes.
No easy answer, sorry. You have homework to do and experiments to try, but you'll learn alot in the process.