subs humming when cars off

If the car is not running and the humm you hear isnt rf bleed-over, then the humm is coming from your HU or your amp switching amplifier section. (me thinkests)

Can you try grounding the shielding on the RCA cable at both ends and see if this eliminates the hum?

Try grounding to both the amp body and the vehicle chassis.

Do you have the gain up all the way on the amp? If it's set on 11, turn it down a bit.

Perhaps the OP could clarify what they mean by "off" and "on?"

Engine running vs ACC?

Get down to basics........

Gozinta.....  Gozouta........

Disconnect remote turn on and see if they still hum.........

Put remote turn on back to the amp and disconnect RCA inputs to amp.......  still huming??

Measure another amp of some kind with a MULTI meter on AC......  looking at inputs and outputs.  I have never done this, I am sure most of the guys here have looked at them with meters and oscopes......  but its a cheap way to pick up some experience with what type of voltage is going in and what you can expect out or a working or not so working amp.

What should be where and when depending on the state of the system......  on or off.

Good ol poking around.

So many ways to test......  but I am thinking you understand pieces and parts......

If you disconnect the RCA's from AMP.....  it goes away.  Well you are on the right track but I am think NO.....  not a bad amp.  It means you are drving something into the amp and it is putting it out the other side.  Sure, there may be some type of issue with the amp, but I wouldnt go that far yet.  Get an IPOD, jack into the sub with a patch cord and turn it up a little.....  any hum still??  Or just some good bass.......

Or, get a batt, head unit of some kind, yank the amp and set it up in the garage and do a test or two......  no hum.....  good amp.

My understanding of a ground loop is that one end of the electronics is at a different potential than the other end.  So ground at the head unit is different than ground at the amp.  Ground loops have both an AC and DC element usually..........

I would get a mutlimeter and take a look by measuring with DC and AC between the two ground points......  if you get a reading......  that may be an issue.

Its like having a concrete floor in the back of the house that is not level with the concrete in the front of the house.  And you are trying to set up bunk beds......