I have an older Rockford Fosgate 500.2 unit and I had some questions about setting the gain. Everything I have read shows to take the RMS wattage X the ohm then get the square root of that. My only problem is since my amp is technically a 2 channel I am wondering how I do this with it in bridged mode.

That wouldn't normally be an issue aside from the fact that this amp also has a left and a right gain control. Now the amp is 500 RMS x 1 at 4 ohm bridged OR 125 x 2 @4ohm stereo. How would I go about using a DMM to set these gains as close to perfect as possible?

sqrt(125*4) = 22.36V

sqrt(500*4) = 44.72

44.72/2 = 22.36V per channel

I.E. Using the DMM to level match each channel to 22.36V is exactly the same as setting the gain to 44.72V for the bridged pair of channels. So all you need to do is set the gain for each individual channel to 22.36V and you will have also set the gains for 500w bridged @ 4ohm. Normally I wouldn't recommend the DMM method for setting a gain and would instead suggest setting it by ear....but since you have to level match the channels, this is one instance where the DMM will come in handy.

What level test tone will you be using? Don't use 0db or you'll be terribly upset with the lack of output with music. I'd suggest using a -6db test tone. The DMM method is far from perfect for setting a gain and doesn't guaranty your gain is set properly. After you listen to it I would still suggest you fine tune the gain setting my ear....just go back and level match the independent gain controls with the DMM to the same voltage.