03-22-12: Dpg3
Impedance: 7 ohms nominal, 4 ohms minimum
Got it from this review.
Wrong speaker. That review is for the M-3, a bi-polar 3-way from the lat '80s. He's asking about the SM-3, a front-firing 10" 2-way made 1982-4.
An inexpensive ohmmeter from Radio Shack would answer your question. I read on another forum that the woofer is a 4-ohm unit, but that doesn't determine the overall impedance of the loudspeaker system. Put the ohmmeter's probes on the two speaker terminals and you'll get a good idea of the nominal impedance.