Ok, I've got a transistor Markbass combo, delivering 500 watts at 4 ohm, no 2 ohm setting, it's not an Ampeg after all. It has an 8 ohm 15" onboard speaker and I add - parallel of course - an 8 ohm 2x10" external cab, so I draw speaker power with 4 (8:2=4) ohms, so far so good. I have another Markbass 8 ohm 12" speaker and always wondered how it would sound if I added that - again in parallel - to the onboard 15" and the external 2x10". I understand that I will then be drawing with roughly 2.7 (8:3=2.67) ohm from a 4 ohm amp, is that an issue if I ALWAYS play the combo at less than half (more like 1/4 or 1/3) of its potential volume? I'm no expert at all, but is the difference of only 1.3 ohm life-threatening?
Of course, mixing a 15", a 12" and 2x10" speakers might sound real crap (or make no real difference at all), but that is another issue. I've always had a penchant for combining different speaker sizes, in my heyday I ran 2x18", 1x15" and 4x10" together (and never had issues with not being heard well even if I didn't play loud).