Hey Geno,
 I've read somewhere in these forums that 2.15 etc High Output MM carts are fine with the ZP3. I've rolled the dice on a soundsmith zephyr mkIII es that I should have in a few days. I will let you know how that works out.
 I plan to run the ZP3 thru the Zstage / Zrock2 as I do now. That should, hopefully, solve any gain issues. Steve D recommends a higher output MM cart for the ZP3...so, again, we'll see.

I've used a 0.5mV cart with my ZP3 and CSP3 and it worked well except for he extra noise due to running the CSP3 so high.  I did this while waiting for my ZMC1.  The 2.5mV cart should work with the ZP3 and ZSTAGE but you might get more noise as well.  I've posted several times that the ZP3 only puts out about 1/2 volt with a 5mV cart and no one has contradicted me.  Since Steve's amps generally require 2 volts to come to full power, the ZP3 is still marginal without an additional gain stage before the amp.

Geno,  you should def. be able to get that hum down to barely audible levels.   You will have to look at how your grounding between the TT and ZP3 are and where the ZStage is connected.   If you have the ZStage in between the TT and ZP3,  try connecting the ZS to the ZP3 outputs and then ZS to amp.    If the ZS is already connected to the ZP3 output then I would look closely at TT cable routing to the ZP3 and make sure the ground wire from TT is tightly connected at both TT and ZP3.

I dealt with TT related hum for years.  Grounding was not the issue nor was shielded or non-shielded interconnects.  My solution was to move all TT related ICs away from my power outlets.  Power cable proximity makes no difference but the outlets sure do.

My Zephyr MIMC is very low output and the hum I originally exp when using a SUT was due to the grounding techniques used. Once I got that sorted out,  I did find that cable/table  proximity to noisy sources like outlets, eqp. transformers, unshielded power cables, etc..  would make a difference in 'picked up' noise.   I don't think the hum is inherent to your cart and 2.5mv is a pretty healthy output but like anything it seems,  there are always exceptions to the rule.   Play around with placement.   Also,  if you have any 'shorting' plugs laying around (or have an easy way to short the inputs to your ZP3,  then you can easily tell if the noise is being picked up between the table and ZP3 or if it is entering the system down stream)….