Well, anyone who puts one 8 ohm driver in with three 16 ohm drivers is making a mistake to begin with. Unless there's a crossover involved, all drivers should usually be the same impedance. They should also all be the same make and model. If they were all 16 ohm drivers, then there are two reasonable possibilities, resulting in 16 ohms and 4 ohms. Throwing in an 8 ohm driver throws everything out of whack. The first thing I'd do is try to get a replacement driver from Carvin, or magnalab, or whatever, so that they're the same. Might be expensive, but cheaper than 4 new ones, I think.
As for the multimeter, remember that it's measuring the DC resistance, not the rated impedance, which is an AC parameter. For an 8 ohm speaker, the DC resistance might range from 5 ohms up to nearly 8, and still be fine.
