the_lhc:
rendu:If you purchase a good expensive speaker with extended frequency response, you would want to use all its potential and therefore extend the bass through the room better than with a subwoofer. Specially for music you want to use the full potential of the front speakers and only leave the last octave for the subwoofer. This will give a much more natural result for music, this is why stereo amplifiers do not have normally a subwoofer pre-out and a crossover frequency.
I'm reasonably certain that, as far as AV amps are concerned, the "large"/"small" and crossover settings are ONLY used where an LFE channel is present, ie with a 5(or 6 or 7).1 soundtrack, for 2-channel (ie music) these settings are ignored and the full frequency sent to the front speakers, which is why some subs have high level inputs as well, so that they can still be utilised with music. So in actual fact if you have "good expensive speaker"s what you set these settings to, makes no difference to the music reproduction.
That's certainly how it works with my amp at any rate.
I do not know about your equipment but in all the ones that I had, when you set the speakers to small it will not send frequencies below 100Hz to those speakers for any source. Regardless whether you are watching movies, TV, or listening to music. This is what the manual of my 2 receivers say. Maybe your works different, you would have to check. Another way you can check is by disconnecting the sub and switching betwen large and small. I have tried that and I can assure you that when you set them to small in my system they are set to small for EVERYTHING.