OK, I just figured it out and fixed it earlier.
I'll lay it out here so that anyone who searches later can find this and not make the same mistake I did...
In the GMOS-05 installation manual that comes packaged with the harness, there'* a section for installation of an aftermarket radio without an external amplifier.
One of the steps that they give you is to "connect the blue/white wire from the radio to the 14-pin connector and also to the 32-pin harness." DON'T DO IT!!!!!
Most likely (and someone can correct me) this bypasses something in the new HU and causes you to lose all your low range, subjecting you to frustration and shopping around for new components when you probably don't need them in the first place.
Anyway, I disconnected that blue-white wire from the GMOS-05 and taped it up and all my kick came back.
So, in short, if you have a blue/white wire with your new HU but no aftermarket amp, don't pair it with the same wire in the GMOS-05. You won't need it.