If by this you mean using a splitter at the speaker out and connecting one end to a sub, yes it would work, provided the amp on the sub accepts speaker level output. If it does not, you might need a speaker/line converter like this one. Generally you would have a slight drop in quality if you're using the speaker level output for your sub...
a local manufacturer in mumbai - dnm - has brought out a powered subwoofer. this has the option of routing the speakers from the amp through the sub. the spkr cables from the amp have to be attached to the sub and another pair from the sub will lead out to the speakers. the sub can be switched on or switched off during playback. this doesnt hamper the output in anyway. believe this is what you are looking for.