Well ideally you should get speakers that match the upstream components, but what I was talking about was for convenience and will apply to nearly any powered speaker or poweramp (with variable gain) and passive speaker. Think of it all this way: a passive speaker will have a gain knob, while most headphone amps have a gain switch. They're not exactly the same throughout but just to illustrate what they're for, the switch on amp lets you use normal gain on an efficient, low-impedance Grado SR325 and switch to high-gain for an efficient, but 300ohm Sennheiser or 64ohm, low-efficiency AKG. The variable gain however lets your speakers work with a wider range of possible sources - it could be an iPod through the earphone out, or the fixed 1-volt line out; or a CD Player with a fixed 2v line out; or a preamp output from a CD Player with an integrated variable output (like a Marantz), an integrated (head)amp (like a Musical Fidelity X-Can V8), or a pro-audio (studio) console. All these have different voltage ranges, whether fixed (like the 1v iPod or 2v CDPlayer) or voltage ranges - my car's Pioneer receiver can do up to 6volts while my Alpine goes up to 2v.
Now, a correct gain structure does a few things. First, from a signal fidelity standpoint, it as much as possible prevents clipping as well as distortion, allowing both the incoming variable level signal and the gain on the amp to work together (it's less tricky with a fixed input signal, since you only set one gain to match on the active speaker, but personally it's a pain in the ass if the knobs are on the amp installed at the rear of the cabinet.) The other thing is for convenience - you should only be manipulating one knob when you want lower volume. If for example you go from a normal gain Mahler recording to the Red Hot Chili Peppers' high-gain Californication,* then ideally you should be able to lower the volume on the preamp (since it's the one closer to you). Get the gain structure wrong, and you'd have to reset them again and again, because while you can dial down the volume to your comfortable listening level, dynamics can get too compressed or you get hissing in the background if one of them is set too high.
A general, quickie way we used to do it in car audio was to set the receiver's/head unit's volume to 3/4, then increase the amp gain until we hear clipping, then dial it back down to a cleaner level. Those competing for SQ events might whip out more sophisticated measuring equipment even just for the gain, but that's where it all starts, especially when you have a different speaker type handling a limited frequency range each pair of which has its own stereo amp (or mono on the sub.) Get this wrong and when you change the volume, the frequency response can have a drastically difference balance - the most usual result being the high frequencies might get too loud compared to the rest at lower or higher volume.
*Given this extreme example though I actually just lower the gain on the media player if I'm listening from my computer
