David, for about five years, I ran the van den Hul Frog cartridge in my system. At 0.65mV of output, the Frog seemed to do best in my system at about 50dB of gain. As you would expect, I have found that my Dynavector XV-1s, at 0.35mV of output, requires more gain. In my system, the Dynavector cartridge does best with gain of 55dB. The XV-1s is an exceptionally dynamic cartridge and, in my experience, does not require quite as much gain as you might expect based strictly on its output level.
The optimal phono stage gain for any given cartridge will vary a bit from system to system, but, particularly given that the Shelter 90X has an output level consistent with my van den hul Frog, I thought you might find this information useful in evaluating the compatibility of these cartridges with the gain provided by your modded phono stage.
Do you know how much gain is provided by your ARC PH-3?
