Over the weekend I tapped into the stock wiring harness on my B7 A4 w/Bose in order to add a sub/amp. Since the signal coming out of the head unit is line level, I tapped into there in order to get a low level signal direct to my amp. I tapped into pins 21, 22 and 7 (the Audi uses a common ground) of this wiring harness:

The problem is I have to have the gain turned all the way up on my amp to get any kind of output from the sub. I figure the line outs from the stock HU are not likely to be as good as an aftermarket HU. I am using an MRV-F345, with channels 3+4 bridged, and have the gain set at 0.2V. I know this amp can drive the sub (11KV.2) much louder than it is as I have used it in the past.

I know I can get a line driver, but don't know that I need one yet. Where should I start testing with a DMM to see if something is wrong with my connections? Should I first test at the connections on the wiring harness where I spliced into in order to figure out the voltage coming out of the stock HU? and then check at the end of the RCA to see if there is a difference? What should I have the DMM set at?