Best investment; cartridge or line stage?

I think this is the wrong question!

The reason is simple: the tonearm's ability to properly track the cartridge far outweighs what cartridge you have. The better the arm is at this, the less differences you'll hear between cartridges assuming they are set up properly.

(FWIW Dept.: I use LPs I recorded as reference. This is a very useful tool which I recommend to anyone who wants to create a proper reference; being there when it was recorded is valuable for knowing what how its supposed to sound.)

Once you have that nailed down properly then I'd look into the phono section. A bit of a teaser tip: if the phono section has a 'cartridge loading' switch for LOMC cartridges, the designer is likely unaware that the 'cartridge loading' resistor is really for the benefit of the phono section rather than doing anything beneficial for the cartridge (it impairs its ability to track properly). There is a simple phenomena at play that you learn about in the first week of electronics at either the technical school level or college level. Something to think about...

@elliottbnewcombjr @ghdprentice thanks for gleaning my questions' true aim here. I've had a few discussions with dealers and cartridge was the overall suggestion. I like to check in with my people (aka audio heads) before I make these kind of decisions. Elliot I appreciate the link.

@tweak1 Another cartridge vote. Thanks for chiming in. And thanks for the reminder about enhancers, it's been a minute.

@atmasphere I like the advice. I have to admit that up to this point I have not dived into the details of turntables. (grammar sounds wrong). Replacing tonearms is at least two steps away from my comfort level today. May have to do the work on the topic later. 

OP, what is it about the sound of your current setup that you would like to improve? I find that a clear objective greatly streamlines the decision-making process.