FALSE!Sound is a waveform, and as such the negative trough will produce the same bass as a positive peak.
Reverse polarity will not affect bass reproduction, although it is advisable to have correct polarity.
8vRicey - I think you owe 16vGTI Paul an apology!!
What he says is totally correct. If BOTH of the speakers are wired wrongly, it will have little or no effect, however, if only one of the speakers has it's polarity reversed, this will definitely have an adverse effect upon the percieved bass output.
Think about it logically - 2 speakers working in unsion, be it creating a negaive or posative wave will compliment each other, whereas 1 producing a positive pulse and the other a negative will cause cancellation of the wave.
Therefore fading from left to right IS the best way to diagnose this issue, as you'll only be listening to one pulse, without the influence of the other.