low power at speaker

I have a 2002 Ford Expedition with factory mach audio system, which includes a head deck, 6-cd changer and sub-amp with small sub-woffer. I replaced all 4 speakers in the f/r doors. I hooked them up to a JVC 4 channel 550w amp for the doors... all works well. I am using the factory head deck but the sub amp located in the rear on the passager side fried a few weeks ago. I removed it and the factory sub-woofer and relaced them with a Boss Mono 850w (250rms) amp and infintity 10" sub-woffer and box. Because the factory head does not have low outputs (rcas) I spliced in the inputs to the Boss on the left & right rear on the highlevel input of the JVC. When activated the 10" infintiy bumps one time (like it should) and a very low output happens. I even wired it in to the JVC removing the rears and running with front door and the 10" in a bridge mono mode, same thing happens.

I first thought the speaker must be damaged (due to open box reduced price) but the local Best Buy says it's find. I would like to use the OE ford had used for the sub-woffer only it seems to only have one input verse two.

questions: will one input hurt the amp? can the head deck be pushing not enough current? I do have 5.6 volts at the boss input for both channels currently using.