REL studio III hum/rumble

I am using a REL studio III with parasound monoblocks. I am using high level inputs. The speakon
left and right are connected to each monoblocks red terminal. The ground is connected to left
monoblocks black terminal. I am using the monoblocks bi-wire posts to connect the sub.

If the amp is in standby, and the sub is switched on I get a huge rumbling sound. This rumble
does not come if the amp is switched on. Another point to note is that the rumble does not occur if the coarse cross-over is at A setting. Also the rumble gradually builds up slowly to crescendo
once the sub is switched on.

Any comments and suggestions are welcome.