[quote user="Andrew Everard"]
But there is an argument for using active subs fed from the speaker outputs of the stereo amp into the high-level inputs of the subwoofer. It's a phase thing, basically - Richard Lord, founder of REL, once gave me a very detailed explanation why it's best at a Gramophone awards dinner, but as it was an awards dinner I'm afraid I couldn't remember the details the next morning, let alone now...
But no, this kind of connection doesn't put any significant extra load on the stereo amp.
Thats what always got me. you are using one speaker to drive two outputs (stereo). Sure, you stereo image is kinda irrelevant at sub frequencies, but the relative phases of the bass between the two channels most certainly is not. Picture this: You have a stereo delay on a bass guitar sound, giving it that sort of 'behind you' sound.. ish. Anyway, the waveforms are 180 degrees out of phase with each other. On a normal 2 speaker setup you get the bass. On a setup where you simply add the two channels together and remove the treble to produce the bass you end up with... nothing.
Of course, this is an extreme example and I'm not 100% sure how 2 channel audio is turned into a single channel for bass... but I don't see how problems with waveform phases can be got around without using multiple bass drivers and two amps inside the subwoofer.
I think I'm lacking some fundamental knowledge about how a sub output is created perhaps...