Ever wanted to know, what your phono cartridge sounds like dead flat??

audio2design, Reading your post, I am not quite sure what you mean.  Are you talking about the effects on frequency response of cartridge loading at the input to the phono stage (resistance and capacitance, mainly)?  If so, those primarily can throw off the hf response for sure, but aren't we assuming that loading is proper in this case?  I was.  After that, it's all about the accuracy with which RIAA pre-emphasis was imposed on the music signal during the making of the LP and then how accurate is the de-emphasis imposed by the phono stage circuit.  Any tube or transistor is inherently ruler flat in the audio range.

Without either accepting or doubting your premise, I thought the premise of the OP's post has to do with RIAA. Period.