Should I use an external Phono Preamp

I have swapped out tubes. I am using Mullard NOS tubes in the phono pre-amp stage and Telefunkin and Siemens in the line stage. I am not complaining about the sound at all. Just always looking for improvement. I may try a new tonearm cable first and then test a SUT.

Quit while your ahead- keep a couple of different cartridges.

With your setup, it would make sense-everything else is in place. You can swap carts when you want a different flavor.

Get your stable in order -Hana, Lyra, AT, etc.

Have a "cart of the week"

The built in phono section of any preamp is not going to compare to an excellent external phono stage.  That Luxman E-07 is very highly regarded and extremely flexible offering adjustments for both MC and a rarity, MM cartridges.  It is not a question of whether the Luxman is better than the Mac, it is a question of how much better.

After looking into the phono stage capabilities in C12000 (which I initially missed thinking it’s equivalent of any basic phono they include with their integrated amps), it is a highly adjustable tube design. It’s supposed to be of very high quality. 
Is the solid state Luxman better, or is it simply a different flavor…solid state, luxman sound vs tube McIntosh…will McIntosh linestage section homogenize the sound of this or any other external phono stage and minimize the differences? Only way to know is to try.

The phono in C12000 is apparently really close to the performance offered by the now discontinued MP1100. 

As to replacing nordost Valhalla 2 phono cables with some other cables first and then testing the SUT…I think the SUT is going to give you a much better ROI. Again…trying it is the only way to know.

I would also look at a cartridge upgrade before the cable change. 

@mooks005 - Have a look at the US Company TTVJ audio, they offer a shipped home loan of an SUT.