MM, if before you tried using plastic to insulate the outer shield on the cable from making metal-to-metal contact with the sub input, a significant point would be if the hum stopped when you pulled the connector part way off, or even all the way off. If it didn't stop, then it wouldn't be a ground loop. I'd suppose that the hum did stop, but you weren't able to insulate it well enough when you tried the plastic and put the RCA all the way in. If so, the ground loop isolator should help.
No, it certainly isn't normal for a second sub not to affected by the ground loop, unless that sub wasn't grounded, i.e. didn't have a 3-prong plug into a 3-prong outlet. One thing to keep in mind is that only God completely understands the mysteries of the ground loop and these things can be hard to trace.
