Grounding a Powered Subwoofer Causes Volume Increase

I have an Audioengine S8 powered subwoofer.  It uses an IEC C7 connection for power.  That means that it does not directly connect to ground. Since I was using Earth-Ground boxes for my Audioengine HD6s, I thought maybe the Subwoofer would benefit being grounded. Audioengine stated that any analog amplifier they use is single-ended.  The sub is a class D amp, but that doesn't mean it isn't analog.

Unfortunately, there are no banana plugs on the sub.  The only in/outs are USB, Stereo Mini-Pin and RCA. According to the instructions the sub only needs one RCA input to receive audio from the source; the right RCA. However, I am using both the left and right RCA inputs. 

Using a voltmeter, I was able to determine that the left RCA terminal could successfully be connected to ground. I used a 16awg silver-plated (not tin) copper wire and wrapped one end around the left RCA input.  I connected a spade on the other end and connected that to my power distributor's Chassis Ground Port.  When I did this, the volume of the subwoofer increased significantly.

Why?

Not that it's a bad thing to have more volume on my sub...it just doesn't make sense.  I have one thought, and it's probably wrong, but it was the only thing that registers.  I read somewhere that with subwoofers, an increase in signal voltage will increase the volume the woofer produces.  Which is why it is best to use both RCA inputs as doing so doubles the input voltage.  The power distributor's chassis ground is connected to the Earth-Ground boxes that are attached to the powered speakers. I tested the signal produced on that line and it fluctuates at .199 - .200 volts. Is it possible that once I connected the sub's left input RCA to Chassis Ground, that it is borrowing that voltage and thus the volume increased?  I cannot imagine that grounding to chassis would suddenly cause the audio signal to increase in volume.