Lewm: "Note however that the RTP is rather unusual in that there is no second gain stage after the RIAA (It's not needed because that hybrid dual differential cascode at the input develops tremendous gain by itself); nor is there a cathode follower at the output of the phono section so as to reduce output impedance to drive the attenuator and the linestage section."
Yes, that phono stage has to work in tandem with the line stage, ie, approximately 50k input impedance (and whatever capacitance in the internal cables), whereas many phono stages with cathode follower are to isolate the RIAA from outside influence and can work with external line stages or loads.
Sidenote: the Manley Steelhead is a variation of RTP circuit (SE not balance) with a White follower so it can drive anything.
Lewm: "When I installed a similar hybrid cascode gain stage at the phono input of my MP1, all I had to do to maintain correct RIAA was to change the value of the first (series) resistor in the MP1 RIAA network, to correct for the change in the output impedance of the gain stage."
I agree that having a series resistor makes tweaking accuracy easier. Since you only have to adjust one resistor, taking the output impedance of previous stage into account, the rest will fall into place. I guess what the purists object to is the high value of the series resistor but the higher the value the less affected by the first stage's output impedance and less fussy to deal with. It comes down to how obsessive you are about such things.
Anyway, looks like you're having fun with your mods.