are some phono stages more resistant to hum?

Dhl,
To take your first paragraph, ALL cartridges (except for a very few, among which are some early Deccas and probably some others of vintage origin) are inherently balanced. The typical modern cartridge does not know or care which side of its coil is referenced to ground. Since historically 99% of phono stages were SE, most cartridge makers label one of the two pins per channel as "ground", just so that the two channels will be in phase, I guess. When we use an SE phono stage, we then treat the signal as SE.

But I did think some about your other point, whether connecting the shield to the negative phase would convert the signal to SE. This only comes up if one is using a cable that was built for handling the signal as SE, but now we are connecting that cable to a balanced phono stage. If the plug is RCA, then the negative half of the signal would have to be carried on the ground side, as far as the phono stage is concerned. As long as the "ground" side of that RCA is connected to the negative phase of the balanced circuit (and NOT to chassis ground, as is the case for your PS Audio), I don't think it would make any difference that the shield was also connected to the negative phase (it would not actually be grounded and so would operate as an unshielded cable). The shield is typically only connected at one end. But this is really a hypothetical situation; if one has a balanced phono section, one should use a balanced cable (two wires for signal, one pos one neg, plus one wire for ground) terminated with an XLR plug.

I know nothing about your PS Audio GCPH, but I take your word for it that you are listening in SE mode. Sounds like it is not a true balanced circuit, if the RCA jack is connected to chassis ground. Or if the circuit is balanced, they don't want you to operate it as such.

thanks to everyone for the responses. It is definitely hum, and not tube or other noise. I suspect that Dhl93449 correctly diagnosed a ground loop. How to solve it, in an apartment with exceptionally noisy wiring (even a flourescent light on the same circuit) and few outlets, is an open question.

Noise issues with phonostages are often quite unpredictable and hard to figure out.

While it might be the case that certain gear is generally more susceptible to noise problems, it can also be the case that it is only more sensitive in certain applications; e.g., sometimes the problem can be cured by moving the location or orientation of the phonostage or interconnect cables.

I have a preamp with a built in phonostage that, in my setup (preamp on a shelf two levels below my turntable) I have hum issues, whereas that same preamp in another system is dead quiet with a phono cartridge that has a .05 mv output (my problem is evident with a .30 mv cartridge).

Hum generated from the power supply of the phonostage itself might be a bigger issue with tube linestages, but, again, when things are right, tube stages can be essentially dead quiet (like my stand-alone phonostage).

I have heard both solid state and tube units that had noise issues. I don't think "type" tells you that much about proclivity to be noisy--one has to actually try the unit. In any given application, a unit that is otherwise known to be quiet may behave badly. For example, a friend's very expensive Boulder phonostage buzzes badly from over the air interferene (from lighting) where a tube unit we tried is quiet (Boulder is NOT known to be prone to ANY kind of noise problems).

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.

All phono cartridges made today are balanced sources. None are single-ended. You don't need a center-tap to be balanced- that is a common myth. I don't know of any microphones that have such a thing- my Neumann U-67s sure don't. Neither do Shure SM-57s- these are mics at far extremes of cost and performance.

Any tome arm that has 5 connections (stereo signal + tone arm ground) can be run balanced by replacing the interconnect cable between the arm and preamp.

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.