How are phono cables different (related to hum issue)?

The problem is not so much the cables as the gain. The tremendous gain in a phono stage, from millivolts to volts, means any noise whatsoever is large relative to the weak cartridge signal. So the cable can be really well made, and one bit of oil or dust on the connector, any little thing, and you can get noise. Sometimes an incredible amount. I've heard radio station broadcasts you could make out the music! Then you touch something, poof! Gone. Sometimes never to return. Other times.... 

It may turn out that your "better" cable is indeed the culprit. I had one that had been used for years just fine develop a noise that took forever to track down to a weak solder joint inside the RCA plug. Unfortunately this is one of those things all you can do is try one thing after another until you either figure it out or give up and buy another cable.

I would start with what Keith Herron suggested to me, plain old alcohol on a clean cotton cloth. Because sometimes even contact cleaner/enhancers leave a film that can cause a problem.

Lack of shielding is a big thing in boutique interconnects. Not all, of course, but yes, that makes a big difference. This can add some capacitance, but with short runs and high impedance at the preamp this should be minimal.

Try out the DH Labs silver sonic line for a good shielded cable.

If you need it, you need it, but shielding closes down the sound.