the pass-thru, if i remember correct, is just rca jacks on the side of the amp that are used as outputs. they enable you to add another amp because it acts like a splitter. you could just hook the 400.4 amp up, then use the pass thru to send the signal to the 800a2 amp, set the low pass filter on the 800a2 to 125hz or lower. this will give the bass, but you have no sub woofer control. some amps come with an external subwoofer control which is a knob that you can put up front to adjust the bass or if you don't have this, just set the gain on the 800a2 to a level that you like. i have sub control and i rarely adjust it cuz my gain is set to a good level. hope this helps.