Measuring impedance with multimeter

Speaker impedance is expressed as an average in ohms; impedance varies with frequency of input signal.

If you connect your speaker to a signal generator and run a frequency response graph for the entire audio spectrum (generally 20-20,000cps) you can see on the graph how the impedance varies quite a bit (from maybe a low point of 2ohm to a high of over 20ohm) over the frequency range. The mfgr of your speaker states it is 12ohm but that is an average of the low and high impedance shown on the graph. You should connect your speakers to the 8ohm outputs on your amplifier.

I have never heard of a 12ohm speaker. Guess you need to get in touch with the mfgr. Good luck.