What MC cart would you use with this setup?

MC - Audio Technica ART9
MM - Ortofon 2M Black

Hana SL mc cartridge is a contender - $750!

The AT-33PTG/II its out of production but are still available at a reasonable price. It has a Shibata stylus.

To get more out of any cartridge (proper VTA is important with 2M Black among others) I would recommend this for your Rega: http://www.michell-engineering.co.uk/tonearms/new-rega-3-point-vta-adjuster/

since you like the 2m Bronze, why not go for the Cadenza bronze?   Another alternative is to send your 103R to Soundsmith for a ruby cantilever / line contact stylus refit.  I have personally tried both of these.

You need a better turntable.

Thanks everyone for the recommendations, I'm not familiar with a few of the cartridges you mentioned, so will be checking those out!

Noromance - when you say I need a better turntable, is it because of the plinth? (everything else has been upgraded...the RB808 tonearm, etc.)

Checked out some of these so far and the Ortofon Cadenzas look pretty interesting, would be nice to find a place that would let me try a few carts before buying, (Minneapolis). That ART9 is compelling, may have to delve into the AT world. Have had that Hana on my radar as well. May look into modding the Dennon, too. I have not been into this hobby for a long time, so appreciate your advice!

Any other thoughts on the rest of my setup? Am curious about whether my TT is a weak link/mismatch when compared to my other components. Still learning. Thanks all!

So...replaced the modded RP1 TT with a Michell Gyro SE, and really like it so far. Still using the Ortofon 2m bronze, will let the new table sink in, tweak setup etc., then will look for a cart later. Thanks to all for the great suggestions! 

Congratulations! Michell Gyro SE is one of the best turntables at any price. 

Thanks, the Michell is awesome so far, and bpoletti is right on, I was way overdue to replace that Rega plinth. 

Dear  @malmc02: I like Magnepan, really good design.

Now and this is me, before change the cartridge there are two changes I will do it if I was the owner of your system:

change the Aria  phono stage for a better unit because it does not matters that you buy ( example. ) a Lyra Atlas or VDH Colibri the LOMC/MM cartridge must pass through that phono stage and what any phono stage can do to the best is: not degrades the carrtridge signal but this is imposible to experienced because all phono stages degrades that delicated signal So you need a better phono stage, yours is so so but at lower level than averagi and is " destroying " the cartridge signal.

The good thing in your phono stage is that is a SS active high gain stage and when you decide to change it follow in that SS active high gain road design.

The other area where you can have a system improvements is to add a second subwoofer both connected in true stereo fashion where your Magnepan's will works as the " satellite " ones.

When your system will performs at a higher quality levels then you really can appreciate the investment with a better cartridge.

You don't need to be in a hurry about. It's better to fine tunning your room/audio system first. As I said, this is me.

Regards and enjoy the MUSIC NOT DISTORTIONS,
R.