Noise coming from subwoofers and centre channel

I’m a huge fan of Furman power conditioners, but I can almost guarantee no conditioner will solve a ground loop problem, no matter how much you spend on it.

Cheater plugs will work, at a high safety risk. What you really want is to isolate the signal ground that causes the loop, not the AC ground.

While there’s nothing in a Furman or megabuck power conditioner that will solve a ground loop, moving all AC plugs to the same strip sometimes is a solution, but it’s not a matter of any circuitry but rather that the grounds are so close.

The OP has already moved everything to the same outlets.  He's pretty much demonstrated that what he needs is an HDMI isolator.

I tried an IFI defender socket this evening, without success, doesn't change anything.
 

All my electronics are on the same circuit, same powerbar.

The Audience Adept arp1 seems interesting at reasonable cost.

I still don’t have an answer that if the power strip is plugged in via a 3-prong plug and the projector is then plugged into the strip with a cheater plug if that’s still an issue.  If the power strip is grounded I don’t see why plugging a component into it with a cheater plug would be a problem as it’s already grounded through the power strip.  Am I missing something here?

I had what seemed to be connectivity issues with my Pioneer Elite AV receiver and the Elite plasma. 

Replacing the high quality HDMI cable with brand new modestly priced cable solved the problem. I don't think HDMI is very robust to begin with. 

Taking your trouble shooting a step further, star grounding, using a power strip into one receptacle and connecting each component one at a time. Plugging additional power strips into the first strip maintaining the single receptacle power source may reveal a grounding issue. 

@soix Yes, you are missing the reason the AC cable has a ground wire.  It's to trip the fuse.

The ground wire on equipment (amp, refrigerator, any) is there to ground the metal shell so in the event of a fault where the incoming hot shorts to the outer shell the current should flow through the ground wire in the power cord at a high enough rate to trip a breaker or fuse.  The idea is to trip the fuse before a person or pet touches it and electrocutes themselves.

The problem is signal ground wires (shields) can't safely conduct that much current leaving two bad options:

1.  High resistance, which leaves the chassis hot and dangerous

2.  Melts and causes a fire

That's why cheater plugs are in all cases not safe.