NAD 314 hiss and hum?

My NAD 314 has developed both a nasty little hiss and a hum.  I'm wondering

if there are some rules of thumb for tracking down these symptoms.

I use generic cheap (Radio Shackish) RCA cables, I'm not sure if they are 
really shielded. Could that be it?

I do have the ground terminal connected to the NAD, disconnection it makes

no difference.

Any ideas?

Dallas