Grounding a Powered Speaker

I have been into making grounding boxes for power equipment to great effect. I had read where folks have actually attached grounding boxes to the negative binding post of speakers.  I wanted to research this so I read up on it. Apparently this can only be done under very narrow circumstances.  Primarily, your amplifier cannot be of a differential design. That means that the audio signal it sends are sent through both the negative and positive channel in a push/pull scenario. Grounding the negative post in this situation will result in an unwanted load on your amplifier as well as diminishing the quality of sound.

In my scenario, I was looking to ground my powered speakers, the Audioengine HD6. I contacted Audioengine and verified that the amplifier on the HD6 is single-ended (not differential). Not to mention that the power cable used is an IEC C7 (figure eight) connector which has no end-to-end ground.  It loops the ground at the device end and connects only to common ground on your wall socket. That's well and good, but that means the amp itself isn't grounded at all, only the cable is.

So I made a cable that uses a Y spade to connect to the negative binding post of the powered speaker and ran the other end with a banana plug to one of my grounding boxes. Not only did the noise floor lower further, but there used to be a faint "hiss" whenever there was no music playing through them; now that hiss is gone.  I had read that this "hiss" was normal for an A/B class amp; now I am not so convinced. Now it just seems like lazy engineering.

For posterity, anyone searching for ways to improve your powered speakers or remove hiss from powered speakers, first see if they are powered by a single-ended amp.  Then you can ground off the negative pole to improve noise floor.