My thought is I could likely improve the performance of this setup by changing cartridges; looking for more detail I THINK a LOMC might be a good idea. Is this a correct thought?
No necessary, but your new cartridge must be perfectly matched with your tonearm (you must know and learn about calculation of resonance frequency according to cartridge compliance and tonearm effective mass). And you have to think what you are willing to improve, what was wrong with Sumiko and its presentation ?
You can easily buy more expensive LOMC cartridge that in fact will be inferior in comparison to the best MM or MI available for lower price (especially vintage top models).
Looking at any LOMC cartridge you have to check GAIN of your phono stage and loading options (if there are any).
Also you have to check for stylus and cantilever of the LOMC you’re after, never buy conical or elliptical if you want the best quality and long life span, cantilever is also important and aluminum is the cheapest, the best today is probably boron (or ruby, sapphire). Here is why it is important (also for MM).
The life span of the MC cartridge is mainly the life span of the stylus tip, because you can’t replace it by yourself. If the manufacturer can’t replace the whole cartridge for you when it’s worn then you can only ask retippers (bad idea).
With MM or MI cartridges you can simply buy spare stylus even for some rare discontinued cartridges. And those styli often comes with the best profiles and best cantilevers. I have a great luck finding them for my needs in NOS like this, and for practical reason i like MM and MI as much as i like some of the best LOMC in my big collection of cartridges.
One of the best cartridge in my collection with dosen of different original replacement styli (nos) is Grace LEVEL II and Grace F14, it’s so much fun, because the manufacturer made nearly all possible variations from very cheap aluminum with best diamonds (Luminal Trace, Line Contact, Micro Ridge, Utility-4 Shibata) to very expensive and exotic (Boron/MicroRidge, Beryllium, Sapphire, Ruby, Ceramic...) and even the compliance is different. Grace made MC cartridges too, this is the rarest Grace MC in my collection.
If you think MC is equal to the word "improvement" then you’re wrong.
You have to be very careful when you’re choosing MC cartridge.
Same with MM, because most of the modern MM just sucks (with some exceptions).
Another favorite is Stanton SC-100 WOS signature model with sapphire coated cantilever and Stereohedron tip, absolutely killer MM (collector’s series 100)
Modern MM cartridges are limited to few few brands, unfortunately.
Would I be able to reap more benefits from a much better (i.e. more expensive) LOMC or would my current system make such a purchase meaningless?
Modern MC are too expensive and most of them are not better than vintage MM
Hope it will help :))