Setting your gain with a dmm...
http://i76.photobucket.com/albums/j23/steveo200077/RMSchart.jpg
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Sounds to me like you might have a really low preamp voltage, or you guys might not be using a sub preamp out either. My first deck back in the day was a pioneer tape deck with one preamp out. It was 500mV!!! The next one I got was a panasonic cd player, once again only one preamp out, that one was 1 volt! Didn't understand why my system wasn't getting loud all that loud without needing to spank up the gain...
Finally got a good deck, with a good subwoofer non-fading preamp out. Also purchased an audiocontrol epic-150...It was like night and day! I didn't always use the epic-150, but the new deck with the subwoofer preamp out (4v) made a pretty big difference.