Hi Mark,
Your thoughts on the Denon's auto-calibration circuitry are exactly why I tell Axiom buyers not to use it. To do careful room measurements, you'd need a lab-calibrated B&K measurement mic that would cost thousands. That is exactly the type of microphone that is used to measure frequency response of a loudspeaker in an anechoic chamber at the National Research Council in Ottawa, where initial Axiom prototypes are measured.
Anyway, the Radio Shack SPL meter is reasonably accurate--within a few dB--when I compared readings to a professional B&K device at the NRC and an expensive meter we have in Axiom's listening room. But if you want precision--and you do if you are measuring or attempting to alter frequency response in a room or in an anechoic chamber--you ain't gonna get it from the RS meter, or from the microphones supplied or sold by Denon or other manufacturers for their auto-EQ circuits.
Regards,
