Didn't try that Paul...but i did try to gound the unit every which way..the most interesting thing was:

-Common ground for both Head Units
-pig-tailed the common ground to the Sony
-started the car
-horrible whine
-pigtained the Rio to the ground
-whine was reduced to the level that it always was

BUT

-when pigtailed all together BEFORE starting car....
-the whine was HORIBLE and would not go away!!

honestly dont know where to go frome here..so took the rio out and just using Sony HU..which..beleive it or not...soundt immeasurably better than the RIO..

-had to turn down the gains on both amps because the music suddenly became much too loud!

...sub sound much better though!