2008 silverado 4ch and sub amp

I was working on an 08 Silverado for a customer, I was adding a 4ch and subwoofer amp to his factory radio, replacing all the speakers.  After everything was wired up, it was working great until the volume on the headunit was turned up over halfway.  Anything above that there would be no sound from the subs.  Went back through and set the gain on the sub amp with a multimeter, had the 4ch amp gain set pretty much by ear. 

After resetting the gain, it would go up a little higher, but after a couple clicks over halfway on the headunit, no bass.  I used two PAC SNI-35 line out converters to get the signal from the headunit to the 4ch and the summed output of the 4ch to run to the mono amp.  The 4ch never lost sound, only the subwoofer amp.  I checked voltage at the amp when it was exhibiting the problem and there was no voltage drop, the power was constant throughout.

I changed the configuration of the system and split the rca's for the mono amp from the LOC at the radio, off of the front channels.  After I did this, I got the same result as before, but if I turned the fader on the radio almost all the way to the front, I could turn the radio up as high as I could stand with no problems. 

So here's the question...do these new GM radios have some sort of bass attenuation system or is there some other reason for the odd behavior it was showing?

Sorry, I realized I forgot to post what equipment was going in.

4ch - Alpine MRP-F450

mono - Alpine MRP-M500