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.

Lewm:

The cartrigde may be connected in "balanced mode", but when you connect the shield to the minus input, you convert it to single ended. You have to reference the input voltage to somewhere, and for phono inputs its the shield (-). If you could get a cartridge with a true balance output (like a center tapped transformer), with the split (or center tap) connected to the analog ground/shield, then you can have a true balanced input. Some microphones are wired this way. But phono cartriges are not.

For example, I have a PS Audio GCPH that uses a "differential" input pre-amp. The input RCAs are ground referenced via a 100 ohm resistor to analog chassis ground. The two phono inputs (V+) and (V-) are twisted pair from the tonearm/cart with a separate isolated shield. Shield is connected to chassis ground. Is this balanced? NO!!! Because the potential between the shield/chassis ground and the (V-) input is essentially zero, because no current flows in that 100 ohm reference resistor. In a true balanced system, V- and V+ would be the same, only opposite in polarity. In this phono setup, V- is 0.
So while this system may give you some CMR, it will not be anywhere nearly effective as a true balanced system.

To clarify my comments, phono carts are ISOLATED devices, but are not inherently balanced. A true balanced voltage source would produce a symmetric V+ and V- signal referenced to common.

However, I agree they can be run in a balanced configuration with the correct wiring configuration and true differential input phono stages.

Sorry Lewm if I misread your original post. Its just that true differential phono stages are very rare, and neither of the OP preamps have true differential inputs, so using balanced wiring with these single ended input stages converts the system to single ended with little to no CMR advantages.