speaker noise when 3rd channel is used

*That is nothing more than a 10k resistor in line with each pre-amp output and then joined together creating a single input to the amp. The resistors prevent backfeeding the pre-amp when the l & r signals are out of phase. I've tried with and without it. I've tried with only the left or right output alone. The only thing I haven't tried is lifting the shield lead to prevent a possible ground loop or using a ground loop isolator.

All of this testing has been performed on a bench using an Astron 20amp 12 volt power supply. I prefer to test my equipment prior to vehicle installs to ensure everything is functioning properly.                                                       

* my little passive pre-amp also has a 100k audio potentiometer to control subwoofer output level. This circuit is not the issue with the noise. As stated above I removed it and still had the problem. I may use a separate sub amp just to see if the problem goes away.

From the for what it's worth department...

I severed the shielding of the subwoofer pre-amp output. The droning noise disappeared. If you place your ear directly against the subwoofer you can hear a very light drone (possible emi). I suspect the culprit was a ground loop. I'm waiting for my CD5441 to get back from United Radio (they fixed a dim display problem). That stereo does not have the non-fader output tied directly to radio chassis ground. I hope it works without severing that shield. I refuse to use a ground loop isolator as the typical unit has dramatic adverse affect on low and high frequencies. In the worse case scenario, I'll use one of my old school Linear Power amps in mono to feed the subwoofer. That will eliminate the ground loop issue completely. On a side note, this Helix HXA 500 mkII 5 channel amp sounds remarkable! I am accustom to using my Linear Power amps which are as close to minimalistic straight wire as can be found. The clarity is fantastic! I am a "less is better" kind of person. Too often people just keep adding equipment because it looks cool. I prefer to use as little as possible to achieve the sound qualities and volume quantities I'm looking for. It's always worked out well.