2 Channel Sub Woofer, 1 or 2 Sub-woofers?

Hi All,

I am thinking of adding a sub woofer into my 2 channel system. I am looking at REL. Should I be thinking a pair of subs, of should I be thinking of a single sub?

Thanks guys for your ideas on this one.

Not to hijack the thread, but a related question I've been wondering for a while is if it came down to getting one very good sub vs. two decent subs for similar total cash outlay, which would you choose? Let's say one $1000 sub or two $500 subs?
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I tried both options described here almost exactly. One $1500 sub and 2 x $700 subs (same brand used all round) and went with the 2 subs. In my room and for my tastes, the 2 sub option sounded better, integration seemed more "right" and set up was easier - just put one beside each speaker.
I vote for 2ch stereo subs --like above have tried both ways and prefer the subs this way also for Soix if it was me I would get the better sub then save money and buy a second better sub