A true vibrato? As in frequency modulation? Well, maybe not frequency modulation. Maybe more chorus volume modulation. It happens regardless how far I am from the speaker, even with my back turned 15 feet away.
Does this occur at low volumes? Yes, but worse as the amp is turned up.
Does this happen regardless of what note you're playing? Yes
If these are the non-soldered clips, does each of the two speakers have the same DCR when measured separately? No, one measures 6.2 and the other is 5.2 ohms.
Does it happen with each speaker wired up separately, assuming you can run the amp into 8 ohms? The amp can handle 4 or 8 ohm cabs. I haven't tried this as it requires me pulling the speakers out of the cab. I can try, but it will take some time.
I'm assuming all the speaker bolts/nuts are properly tightened. Yes, torqued.