Sorry, mrmoe, I didn't make it clear enough!

Yes, you are quite correct that you should connect both loudspeakers 'in phase'.

What I was referring to are those HiFi nuts who think there is an 'amplifier end' and 'speaker end' of a piece of wire to connect to the speakers! Okay, if there are plugs on it could make a difference - the plugs might not fit the other way around!

ALL loudspeakers are AC, or they wouldn't work - it's just the amplifiers in powered loudspeakers that may be powered by DC. The connection between the amplifier and loudspeaker is always AC.

Applying a large DC current to a loudspeaker can make it catch fire. Anyone who has seen a power amp 'go DC' (usually when a driver transistor fails) will know what I mean.

A loudspeaker requires AC modulation to be able to work - it is basically an electromagnet which pushes and pulls the loudspeaker cone along a fixed magnet to produce the sound.

'Phase' is a phenomenon of AC current. If you were to feed a sine wave through one loudspeaker, and the same sine wave with the phase inverted through a second, identical louspeaker, in theory you should get silence. In practice there are other issues (distance, etc.), so you get a very 'thin' sound.

If you wish to check the phase of a loudspeaker, you can safely use a 9v battery (yes, DC!) across the terminals. Connect the battery one way, and the cone will move forward (the + terminal is on the 'hot' terminal of the speaker); connect the battery the other way around, and the cone will be pulled inwards (the + terminal is on the 'cold' terminal of the speaker).

This also proves that speakers aren't DC!