A turntable or record player ( just to make sure : motor/arm/cartridge combination ) is perhaps THE sterling example of all things technical and human related, usually far from working in concert. The guy that does the motor will say it is a problem of the arm maker, the guy that does the arm will say it is a problem of the cartridge maker - and so on, table tennis game turned into perpetuum mobile. And there may well be a commercial reason of some party involved that will prevent you from getting a fair answer and solution to the problem.
The fact that MisterMoJo preferred MM cart into a MC load/gain suggests this may be one of the worse cases. Falling high frequency response that usually results must have been helping masking some/much of the distortion. As always, prevention is better than ( inappropriate ) cure.
It is either depending on the dealer ( questionable as found out ), getting the table fixed by an unbiased expert ( like myself, do not expect it for free, it is usually a day's work, and that does not mean 8 working hours but usually more, till the thing starts to sound right ) - or the best solution, learn how to align/adjust the turntable by yourself. Minimum requirements : alignment gauge, vertical tracking force gauge, NEW unused test record - and the ability to use all of these.
The list of mechanisms that can lead to distortion in turntables:
1. Stylus worn
2. Stylus improperly ground/misaligned relative to cantilever
3. Cartridge misaligned
4. Vertical tonearm bearing - free play (resonances ) or too tight, making the arm to stick more at some points across the arc than others and/or not frictionless enough to allow the stylus to track.
5. Horizontal tonearm bearing - same reasons as above
6. Vertical tracking force too low to track the recorded signal - usually occurs most at approx 5-10 kHz range, where recording velocities peak and can reach and exceed 100 cm/s - those pesky sibilants and an occasional really well recorded cymbal crash etc. Not even the best carts do not do well here - you need effective stylus mass as low as you can possibly get ( and afford ...; just look at the Grado range prices and you will get the idea, you are paying for lower effective mass and higher precision, basically the cart is the same design from top to the bottom of the range )
7. Antiskating set too high or too low relative to the vertical tracking force used.
8. Cartridge electrical load misaligned enough to cause severe peaking in response that can lead to the sibilance.
9. Phono preamp overload too low, leading to sibilance with hot(ter) recordings
10. ALL of the above can - and DO - interact with each other, in all combinations and permutations possible.
11. THE WORST of them all - record(s) permanently damaged due to anything or combination of 1 through 7. Here only the use of Micro Linear (Scanner, Ridge, Reach, SAS ) or VdH I (or equivalent(s)) stylus tip profile can offer remedy that will sound acceptable - BUT only if the damage to the records has been inflicted by the use of the less sharp stylus profile - which is the only saving grace you can reasonably count on. Styli mentioned are TOTL and therefore expensive, one does not usually start turntable journey with these top styli.
Without access to the turntable in question it is impossible to say what is the (most offending) problem.