Should I use an external Phono Preamp

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.

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"