I recently got hold of a multimeter and was checking the ohms rating on a bunch of 15's I've been stockpiling. Everyone came out to just about the rated impedance except for 3 old JBL K140's, two of which had recently been reconed. All three came out to 6.1 ohms. So I was wondering if age has any effect on impedance with older woofers? I tried the probes every way I could on the K140's, but they always came out to be 6.1.

Age will not affect DCR, only damage can. The fact the two recones measure the same as the non-reconed one could be an indicator of factory spec cones. DCR varies with design.

The chances are that the new cone kits for the K140 are in fact E140 Re-cone kits and if the old one is original k140 how nicely similar they are in DCR which of course has no more to do with nominal impedance than say measuring lumens, calories or BTU.

Temperature rise makes resistance (DCR) go up. Temp rise can make capacitive reactance go up (makeing z go up). Meter calibration changes. Most meters are not very accurate at the low end of the DCR range.

Ok, well, if all that is so, why rate them at 8 ohms nominal ? Also, do I need to be concerned that, if I put two of them in parallel in a 215 box, I'm gonna get a 3 ohm load, which is below the minimum impedance of any amp I have at the moment ?

DCR is not the same as impedance. 6 ohms DCR is spot-on normal for most 8-ohm drivers. It's your other drivers that are "unusual."

You don't use measured DC resistance as the rated impedance. Two speakers, rated 8 ohms each, in parallel, have a rating of 4 ohms. That is what you use, 4 ohms. The rated or nominal impedance is typically chosen from one of the standard values, 4, 8, 16 ohms, and is usually higher than the actual minimum impedance that it is based on.

To clarify some physics: The DC resistance of a wire at a certain temperature is a function of the wire length and the wire size (diameter, also called gauge). It's very predictable. It doesn't matter if the wire is straight or coiled up into a speaker's voice coil. Nor does time factor into the DCR. If the wire stays the same temperature, length, and size, it's going to be the same DCR now or 100 years from now (provided of course that there's no mechanical damage, chemical corrosion, or other changes). Impedance, or the resistance to AC, is observed when the wire is coiled into an inductor. The laws of physics are pretty well defined here also--if you know the wire size, the number of turns, and the diameter of the coil, then the inductance of the resulting coil can be very accurately predicted. And like DCR, if no changes have been made to the coil, its inductance will be the same now or 100 years from now, provided that it's at the same temperature, no mechanical damage or physical corrosion has taken place, etc. I've got a radio in the basement that's over 80 years old. I guarantee that it's air core inductors measure the same now as they did when the radio rolled off the assembly line.

You have measured DCR (Direct Current Resistance). You did not, nor cannot with a simple hand held meter measure Impedance (AC measurement also dependent on the frequency of the AC signal applied). Impedance varies greatly over the useful frequency response of the speaker.