are some phono stages more resistant to hum?

Lloydc:

When you disconnect you TT ground and the hum is reduced, it implies you have a ground loop. You cannot have multple points of ground in different locations, as these will create hum as ground current flows from one location to another.

For example, if your TT base is metal and grounded to the third wire AC ground, and the analog input ground in the pre-amp is also grounded to the third wire ground, you have a ground loop, with the RCA cable shields making the connection.

The reason why some pre-amps have different hum performance may be due to how they reference the analog signal grounds at the inputs. Some are hard connected to a chassis or analog ground, and some are referenced with a capacitor or resistor. The resistor or capacitor provides a high enough impedance to interrupt or reduce ground loop currents.

Differential inputs are useful for CMR, but phono carts are almost alway connected single ended, which means the (-) and shield are common, and you cannot get a true balanced connection to the pre-amp input. So the CMR of the input is of limited utility.

Something that worked for me with a P75 phono stage was running a wire from the ground post of my phono preto the chassis of my preamp

Dear Dhl, If the phono circuit has a balanced topology, then the cartridge may be connected in balanced mode, and believe me there is CMR for the phono signal which does cancel hum along with other noise common to both halves of the signal. You can get a true balanced connection no matter how the cable is built, because the cartridge per se has no true ground, but it is indeed better if the cable provides identical conductors for the pos and negative halves of the signal, and a separate ground wire that generally grounds the tonearm body only. I don't know why you seem to think that is a problem or not available.

Lloydc:
I am wrestling with a similar condition after replacing my ss pre with a tube unit. Are you sure that your problem is a 'hum' and not a 'buzz'? Mine is definitely a buzz (heard only in the tweeter), and my current thinking is that it relates to external circuit noise which is induced into the system and picked up by the tubes and sent through the rest of the system. I must admit that I am early in my process of diagnosis, but nearby lighting dimmers cause similar a similar buzz when activated. Anybody else have comments on that line of thinking?

Rtilden, Check to see whether any of your ICs, particularly the one from tonearm to preamp, are running parallel to or anywhere near an AC cord or a PS umbilical. I had an identical buzz that drove me a bit nuts until I noticed that my phono cable was running near to BOTH the AC cord from the turntable AND the PS umbilical from the phono stage PS. Re-routing it made a huge difference. Sometimes we lose track of what's going on with all the spaghetti hanging down at the rear of our system shelves.