Hi upyourkit,
It's puzzling, but my first instinct is that you might be running Yamaha's YPAO auto-Equalization on the center channel. Turn it off and leave it off if you are running the auto-setup or auto EQ. Do a manual setup, ideally with a Radio Shack sound level meter or by ear if you don't have the Radio Shack meter.
The previous advice about checking each driver in the 150 to make sure each is working is important. Put your ear up close and play pink noise or a wide-range recording. Once in a while the connectors to the driver(s) may get dislodged in shipping.
Leave your Yamaha set to the 8-ohm setting for all your Axiom speakers. The 6-ohm setting may cause it to limit current output.
It's common for sensitivity to vary with different manufacturers but not by the degree you cite. What is the impedance of the old Polk center?
Regards,
