What MC cart would you use with this setup?

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.