Hum Help

I had an interesting case of hum pop up (that I was able to solve) but interested in learning a bit more. 

I have a Technics 1200 Mk5 that is pretty heavily modded. I currently run a Jelco 750D arm on it and have had MM cartridges on it forever, but recently swapped to a Hana SL and picked up a low output Grado Opus3. The table is connected to an ifi iPhono. 

When I made the swap to MC, I started to get a hum on my amp on that line in. I figured it was the iPhono, but I swapped in my Phenomena and it was still there. I tried with both MC cartridges and it remained. I connected the iPhono to the amp using a different rca on a different channel - still a hum. That left the tonearm and phono cable (a Zu Mission). I swapped the phono cable with one from a stock Pro-Ject deck. Hum went away. 

The Zu is not very 'shielded' (?) but I run mostly Zu cables and have never had this issue. I have a long Zu going from the Phenomena to the amp, for example, with no hum. 

Any ideas about what might cause this? I can pretty easily get a better cable, but I'm not sure if that is more like putting lipstick on a pig, as they say, because there is an underlying issue.