There are several issues...What sort of impedance characteristics would cause real concern for a standard 4-16 Ohm rated AVR/amp? And if you have speakers that dip to....say 2 Ohms....then what should you look for if the amp is not declared as unconditionally stable down to 1 Ohm?
- at low impedances some amps become unstable, their circuits just aren't designed for the levels of current required, and the sound is affected - even though they are nowhere near their "power" limits overall
- Current, Current and more current - ultimately low impedances require current - and regardless how good the power amp circuit is, if the power supply (and the wall circuit potentially!) isn't supplying the required current it is all for nought
This means you end up in a guessing game.... you can get some hints eg: if it is a linear / transformer based power supply, the sheer weight/size of the transformer will provide a decent idea as to current capabilities. ( yes my old 58 pound AVR could handle my speakers where my current 20lb one does not.... not exactly scientific / quantifiable, but nevertheless it is a data point)
In a perfect world I want every manufacturer to provide a graph like this one, from the Quad 606 manual circa 1987(?):