Should I use an external Phono Preamp

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

Tube rolling is a great option, but can easily become an expensive investigation, hence my referencing a change to the sonic, via a SUT or Head Amp.

As stated ' There is a US Company offering home loan of a SUT Model', this seems very cost effective to experience a change to a sonic from the Phon' Input.

@mooks005 

McIntosh does this very well. You’d be hard pressed to find anything that would be a sonic improvement or price competitive with what you have.

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.