are some phono stages more resistant to hum?

There are probably a dozen reasons why one phono stage might be more likely to hum with a particular tonearm and cartridge where some other phono stage will not. First of all, lets get the terminology straight; when you say "hum" it implies a 60Hz or 120Hz tone (in countries where the mains frequency is 60Hz). Hum is almost always due to inadequate grounding. By that definition, tube phono stages are no more likely to hum than SS ones. However, true balanced phono stages of either type are much more immune to hum than are single-ended circuits, because even if the ground scheme is not perfect, hum in a balanced circuit is often cancelled by the mechanism of Common Mode Rejection (CMR). The better the CMR (i.e., the more the two halves of the balanced circuit are truly in balance), the more the phono stage is impervious to hum. One kind of noise that is akin to hum is the rare instance where the cartridge will interact with the tt motor, some types of which can induce hum in the cartridge. "Bad" tubes can sure be noisy, but the noise does not usually have the quality or pitch of hum.

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.

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.

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.

The OP asked whether one or another different types of phono stage might be more or less susceptible to hum. In responding, I brought up the fact that true balanced phono stages are much more immune to hum. Then we got off on this tangent. I don't think the discussion was out of line with the intended subject of the thread, but it was prolonged by your insistence that a cartridge is not a balanced source. Anyway, it's all good.