Micah, when two speakers are run in parallel the resulting impedance is such that its reciprocal is the sum of the reciprocals of the impedances of the two speakers. Using 6 and 4 ohms, this works out to 1/6+1/4=1/X, 2/12+3/12=1/X, 5/12=1/X, 5X=12, X=2.4ohms.

Keep in mind that the nominal impedance ratings of speakers don't represent their impedance over their entire range(the 4ohm rating on the M80s is a notable example)and that 2.4ohm result would only apply at a frequency where the speaker impedances were in fact 6 and 4 ohms. Also note that a low impedance doesn't necessarily cause a problem if the power used at the low impedance frequency wasn't high enough to require an excessively high current.

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Enjoy the music, not the equipment.