Rob,
Something I was thinking about while playing with my MM 651 Polks. The MM are a 2 ohm rated speaker, the OEM are 3.7 ohm, I believe. Don't know for sure, but my guess the oem amp is rated for 4 ohm speakers and the MM operate at a lower resistance level, whereby they reguire more power with less resistance, hence the burp sound you seem to be getting.
I'm not sure the net effect on the oem amp either. I never thought about it because I installed a 2 channel, 2 ohm amp that will handle 4 ohm loads, but I specifically matched it to the 2 ohm MM 651.
Anyway, I'd suggest perhaps it's a resistance issue( too much resistance-too ltitle power). I've heard no others trying MM Polks with an OEM amp.
Good luck

Jim