Should I use an external Phono Preamp

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.

I have also looked on Amazon, where I discovered that in the UK, a range of MC Step Up Transformers are offered, which will all be returnable and fully refunded.

Similar should be found on the US site, with the same refund-on-return sales service. 

So, we have some knowledgeable guys testifying that the Mac owned by the OP is a very fine piece of gear and would not be easily bettered except by spending a great deal of money, and yet others suggest that a SUT, just any SUT, might be an upgrade to the high gain circuit of the Mac.  Then we have another respected contributor saying that no full function preamplifier can compete with a separate phono stage when it comes to vinyl reproduction. ("Poppycock" is what I say to that.) But what is a poor OP to do with such confusing advice?