Infinit Qx4 Factory Integration Issues

Ok, so today I did an install for a friend. He's new to car audio and wanted to keep the system cheap so we picked him up two 15" Kicker Comps and I built him a box for it today and wired everything up. Here's where it becomes interesting. It's a 1000W Kenwood amp (500 RMS @ 2ohms) powering the comps wired in parallel @ 2 ohms. Because he wanted to keep the price on the low, he opted for keeping the stock head unit and using a High-Low Input signal converter (speaker to RCA). Well, I spliced in the input signal converter in the back after the factory amplifier and then ran the RCA's to the Kenwood sub amp and then set all gains accordingly. Now, for two 15" cheapy comps, it slams really hard, but only in the mid-upper bass notes. I don't mean high notes, but I mean it won't get really low. It's not the box, because its tuned to 33Hz...my box is tuned to 35 and it gets lower and louder on those lows with my little 10" comps. Basically, it's like there's subsonic filter or something killing the lows being put out because the subs are barely moving when you get below 30Hz or so, and the subsonic filter is OFF by the way.

My first instinct is that maybe the factory Bose system's amp has a higher set subsonic filter to protect the door speakers and since the amp is being fed a signal post-amplification, the sub amp isn't even receiving those low notes. Any way to fix this? The system came out great other than lacking low-end bass. It's a 98 Infiniti QX4 by the way. I know you have a QX4 WOT, so any comments would be appreciated.

Also, oddly enough, I might be getting rid of my car, and the car I will almost surely end up with will be a QX4, a 1999 though, so I'd like to know this for my sake aswell //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif