I'd still like to know if Polk's measurements are in-room or anechoic
Got a link to their specs?
While they're still speakers and impedance and sensitivity are part of the equation, most subs you'll find are going to already be matched to an amplifier suitable of driving them. The watts they advertise, though, become somewhat meaningless since you likely won't get the impedance/sensitivity information from the manufacturer. 4 ohm is a good bet on a sub driver but not necessarily; sensitivity is rarely shared.
Most important is, really how they sound, specs may help you determine some aspects of how to use them; sub frequencies are harder to distinguish by ear vs normal speakers, tho. I want subs that at least can handle 20Hz at reasonable output and distortion levels (if it doesn't dig to at least 20 it's not much of a "sub" in the first place IMO).