Frequency Response

Sorry....speakers

Extension at either end is less important than linearity in-between. Meaning the amount of +/-, or how 'flat' they measure. Personally I'd say at most +/- 3db over it's operating range, and +/- 1.5db being what I insist upon myself.

30Hz to 30kHz is not standard, although it is good. It is also meaningless without qualifying it with +/- however many dB's.

You should do some reading because you clearly don't understand this or how it would relate to sound quality. But generally, a vague spec like you proposed is meaningless. It tells you nothing about the linearity and nothing about how many dB's the response is down at the frequency extremes...

I agree with Plato but would like to add - Specs are just a guideline. Go and listen then make a decision from there. As far as a standard? For what - a desktop speaker or a 7' floorstander? Also +/- db can be all over the entire spectrum and without a graph with a known load you can't just assume it's at the high and low end.

I will go out on a limb and just tell you that 20Hz-20KHz reasonably linear i.e. +/- 3Dbn is fairly standard fora large floor stander. My Focals actually don't extend done to 20 cycles but stops around 27 Hz. I don't miss it but some people like the vibration that a subwoofer typically provides. I find it ludicrous that people can't just give you an answer. It should be noted that linearity is very important but you didn't ask for that information.