Folks are now recommending general-purpose MM/MC phono stages. OP’s specific cartridge is NOT a normal MC. It’s a 0.05mV mega-ultra-extreme LOW output and needs something more specific. He already has a matching 50x SUT (35dB gain), which ideally would be paired to a ~ 45dB - 50dB voltage gain stage at 47K (or higher) load to hit the electrical "sweet spot". Any lower than 47K load will not work well. 40dB gain is probably not enough (without a lot of downstream line gain available).
That configuration should work well; lots of nice options for that. I’ve been proposing an alternative, in the form of a 10x SUT plus an active voltage gain stage in the range of 60 - 64dB, with load of 2K or higher (as @lewm pointed out). It is very unusual to feed a SUT into a full MC stage, but I think that might work nicely here due to the very unusual circumstances. Loading is very crucial here!
No active MC stage on earth of going to amplify that 0.05mV on its own, without the help of a SUT and without rendering a MASSIVE noise floor!
@bdp24 the Herron VTPH-2A specs 43dB or 48dB gain MM mode and 64dB or 69dB MC mode (depending on tube configuration). Were earlier iterations lower in gain? The 48dB MM model would be absolutely ideal for OP with his 50x SUT and a 2K (or higher) loading plug (or no loading plug!). However, I was under the impression that you cannot simply swap tubes (pair of 12AX7 vs 12AX7) to go from one configuration to the other - I thought the RIAA EQ was tuned at the factory for one or the other, so it would have to go back to Keith (or take instruction from him) to properly change config - because the 2 tubes in question are part of the RIAA eq circuit? My VTPH-2A is the lower gain config, 3x 12AT7 and 2x 12AX7. This config was Keith’s typical recommendation. It would be great if I could just swap tubes to change config (if 12AX7's were food I'd be set for life). And I’ve definitely come to appreciate the VTPH-2A’s virtues more with time!