Should I use an external Phono Preamp

If you could acquire the Luxman E-07 with a return privilege, that would be my recommendation.  Your your GME is far better than @dover gives it credit for being. The arm in particular is worthy, so you could consider upgrading cartridges in the future with it.  The Luxman would offer tremendous flexibility for that.  The unknown factor for you is to know if it would be enough of an upgrade to be worth it over the Mac.  You would be the best judge of that by trying it.  Let us know what you decide.

@mooks005 I was wrong about the phono in C1200. I was thinking it’s comparable to what’s in McIntosh integrated amps, which is not as good as MP100. . @dover is correct. 

Your Ortofon has an internal impedance of 5 ohm’s making it a great fit with any of Sutherland’s TZ series pre-amps. I am running the TZ-Vibe and loving it. Saving up money for a Little Loco, or a Duo Loco. 

The Sutherland Dos Locos would be a great match. This is perhaps the only upgrade that I have ever made, from the Duo, that has dropped my jaw!  Everything that reviewers have said I agree with - it finds details that the Duo did not, the bass is richer and deeper, the treble more extended, the soundstage is bigger and the mid range is totally musical.

You cartridge checks all the boxes for compatibility.

Have you tried different tubes in the phono and/or line stage of the C12000?

Also (already mentioned  believe) the C12000 has loads of adjustments.

DeKay