Frequency Response

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.

Please consider that the frequency response should be accommodated by the room in which they will be placed in.