This is not a criticism, just a observation of a trait that has been shared about your latest Phonostage acquisition. In Your Post it is stated,
" overall means it’s not overly coloured like some tube phonos but it has the density and 3-D holographic quality with excellent tonal balance and micro dynamics like some European phonos 2x the price "
I use Tube Designs that are towards the Lean Side in their presentation, I can add an increased Rich Tone through a use of a SUT. I use a Cart' that works with maintaining this attractive trait.
I can add a overbearingly Rich Tone by using a owned LOMC through the MM Input, it is certainly not to my taste, but is a achievable experience, with the Volume Maxxed Out .
I'm confident the SPU is able to make the inherent Rich Tone become quiet noticeable, is this the desired outcome, to add Richness to a Phon' that is selected for it not having a too Rich Trait in how it presents.
I would think a Cart' that enhances the trait that caught your attention might be a alternative way to think about marrying a Cart'>Phon.