Faulty RCA cable, grounding issue or something else?

[Bah]

I recently had to buy a new amp, as my older one went 'pop'. As the newer one had an integrated phono pre-amp, I connected my Dual 505-4 which had sat idle for maybe a decade (yes, preamps do come separately, I just never got around buying a new one to replace the one that was lost when I moved). But after I connected the turntable to the amp, it started to emit a loud ground loop like hum across all the inputs in the amp. After checking the ground wire was connected, I realized I hadn't even plugged the turntable to a power socket. I have experienced ground loops before, but do they occur without power? I also never have experienced it coming from other inputs, either.

 

I tried other outputs in the phono input, and the hum disappeared. Connecting the RCAs from the phono to other outputs still made a hum (and also on other inputs), so it's most likely the turntable. The hum is a lot more lower when only one channel is plugged in, but still loudish. The level of the hum changes when I move the RCA cable. Before I go and open the thing up, can anyone give an advice what would be the probable cause and what should I look for other than the obvious faulty connections and frayed cables? I'm guessing it's a faulty RCA cable, as it's the original and about 30 years old, but as I don't really know that much about these, I could very well be wrong.