Determining cabinet impedance

I recently picked up a used Marshall cabinet. It was formerly a 4x10, but it was refurbished into a 2x12. I know it has 2 8-ohm speakers (G12T-75's I think), but I don't know if they're in series or parallel, meaning either 4 or 16 ohms. I would open it up and take a look, but some of the screw holes look kind of sketchy and I'd rather not take them out if I don't have to.

Anyways, I hooked it up to a cable, put a multimeter between the other end's tip and sleeve, and got 12 ohms. Then I tried prodding around the jack, not expecting to get anything, but it did work and I got 6 ohms. Now today I tried it with a different multimeter through a cable, and I got around 8.

What's happening? Is the resistance in the cable adding to the reading? Or does it come from the fact that my voltmeter measures resistance (I think), which is technically different than impedance? Is there a better way to measure it? Or am I gonna have to open it up?

A speaker's impedance changes, depending on the frequencies it's reproducing. The rated impedance is an average or typical value.

DC resistance IS different from impedance, but it can be used as a general guide to determine speaker impedance. The standard rule of thumb is to measure the DC resistance and multiply by 1.3 to determine the probable rated impedance.

A cable will add to the resistance, but it should normally only be fractions of an ohm. You may want to make sure your meter is accurate and your probes are clean. Also, don't touch the probes while you're making the measurement (a lot of people make this mistake when probing a 1/4 inch plug). Your hands will effectively become a parallel resistance, and they'll throw off the reading.

Anyway, if the speakers are in parallel then you should measure around 3 ohms resistance. If they're in series then you should measure a little over 12 ohms.

open it up,check the specs on those speakers to confirm two 8 ohm units. it's used therefore you dont know what may be in there. If you have shotty screw holes you'll want to fix them, you dont want your cabinet making funny noises due to vibration. Plus you may find crap wiring or other things that need some TLC.

It could be that a switching jack has been used, in the typical Marshall arrangement due to its former life as a quad. Plug a mono cable in and it runs the speakers in series (16-ohm; hence your ~12ohm reading). Unplug the cable and it disengages (or engages) a switch, so when you probe it without a cable you end up measuring only one speaker (hence 6 ohms).

I dunno about your 8-ohm reading though, maybe the cable is dodgy.

and quick fix for shotty screw holes:

q-tips. the cardboard shaft type. Simply cut the cotton head off, insert into screw hole to determine depth and cut to fit. re-tighten screw.