I would think that unless you're using extremely long speaker cables or very thin conductors, the DC resistance of the cable is insignificant. 100 feet of 12 guage wire has a resistance of less than .16 ohms. Double that to just over 1/3 ohm for the two conductors in speaker cable. Typical speaker cables are no where near 100 feet long, making the cable resistance pretty much a nonfactor.
I'm still learning about this stuff, myself, so I have a few questions:
Since the impedence of a driver changes with frequency, at what frequency does your impedence meter measure impedence? Or does it sweep a range of frequencies? Can it measure the impedence of the air in the cabinet? (It's my understanding that, when loaded with the same driver, a horn loaded cab will have a higher impedence than a ported cab.)
When I measure a "speaker line", from a band stage perspective, you are correct, the resistance is minimal, but it gives you a true accurate reading of the speaker and it's impedance load. For instance, his two subs in series might weigh out at the cabinets as being 4.6 ohms or so each, rarely does a speaker ring out at its "rating", usually they are a tad off, over or under, if you are within a an ohm or so, you're usually more than fine. Measuring the impedance also identifies shorts in the line, or opens, telling you that you have a bad cable or it might help you identify a bad crossover or driver. Or if one box reads 4.X ohm and the other one reads 3.2 ohms. You're getting the load on the amp to be a bit more precarious, might cause it to overheat or fail if you run it too hard too long. Older amps with 2ohm ratings can usually handle the impedance mismatches much better than newer amps which have sacrificed construction for price. The PLX is a great amp and could handle it.
Since I do pro sound for a living, I am looking at cable runs of 100-400 feet, the resistance does start to affect it, but again usually if all is good, no big deal. You might have to drive your amp a bit harder. But it also lets me see if an 8ohm box I have 100 ft. away is running at 6 ohms or so (this happens often due to internal crossover networks) and another identical box on a delay line is ringing out at 4 or 5 ohms, that there might be an issue with either the drivers in the box, the integrity of the cable, or possibly the crossover network in that box.
Of course, I understand if you are using it to identify shorts or opens, you could just as simply toss another speaker cord in line and toss the old one in the "don't work" bottom of a cable bin. But to truly know your load in an iffy amp situation as suggested, it's really a good idea to know what you're doing to the electronics...because they could lead you to blow a box too.
I appreciate the idea that you should never mix subs, you'll certainly never get two different models or brands to sound alike if that's the goal. But I have brought in additional subs for a few outdoor shows and it was a better than nothing idea that was ...better than nothing.
