Try disconnecting the ground from the TT to the phono stage. On rare occasions some cartridges work better without this ground. Worth a try.
To see if the hum comes from the phono stage, try using a cheater plug on the phono stage PC and see if it goes away. If it does you have identified a ground loop problem as opposed to other possibles. If that works I would leave the cheater plug in place and relax, although others might not. You do not IMHO need to establish a seperate ground between your phono stage and your pre-amp.
To see if the hum comes from the phono stage, try using a cheater plug on the phono stage PC and see if it goes away. If it does you have identified a ground loop problem as opposed to other possibles. If that works I would leave the cheater plug in place and relax, although others might not. You do not IMHO need to establish a seperate ground between your phono stage and your pre-amp.