I wonder what the "adapter" harness did with/for your sub signals. The OEM sub signal comes from the smallest of the three connectors.
The factory HU and amp is designed to use and control two overlapping signals. There is a Mute signal buried inside the subwoofer signal, thus the amplifier is looking for that complicated signal. If you really have just the stock system and proper adapter, I would think about removing the stock sub amp from the circuit. I don't know how the adapter is sending a sub signal to the back, that would be a possible issue. The main difficulty of the factory Mach system is that Mute signal. It is hard to mix aftermarket parts on either end. If you remove them all then you can be sure of how aftermarket parts will work.
I consulted with Crutchfield about it when I thought I had a handle on what was going on. That was about five months ago, and they knew far less than I did about the Mach system. They had never heard of the factory mute signal, and had no clue of how to eliminate the signal from my stock HU. I learned what I know of the Mach HU from a pro who only works on those. He added internal RCA outputs to mine, five of them.
Since you have a known entity in an aftermarket HU, why not remove the other oddity of the OEM system, that sub amp. With a normal amplifier for the sub, you would be in control of the output. Your current HU is sending out a full range signal, that's why you are hearing vocals back there.
I don't know how the adapter is creating that sub signal from your HU, that's what you need to know. Good luck,