I am not educated enough to answer your question, these are the types of question I would take direct to my Third Party Cartridge Service I depend on.
Third Party Cartridge Services from what I have discovered through investigation and discussion with the Technicians does suggest there is a Channel Imbalance being checked and corrected as part of the Serice on offer.
There are Third Party Services who are happy to show evidence of their work, and some will put out photos of the Readings that are being achieved as part of the refurbishment work.
My reasoning for the purchasing a Phonostage with a L+R Channel Gain Control, is to do with my totally trusting the Designer /Engineer and accepting the options incorporated into the device to deliver as per the designer's intent.
Additionally, as I am usually working with Dual Mono Volume Controls in the Pre- Amp's used and will one day soon have a my New Design Balanced / Single Ended Pre-Amp' Built, to be used with the Modified Power Amp's, which will have a Balanced / Single Ended Input, the option to tweak a loading on a Channel was to me a worthwhile tool, as the conversion to a Balanced Signal Path as the additional option has been projected for a very long time.
I am avoiding the subject of Phon's and Balanced Signal Paths at present.