Dunno. I don't really understand why they include the centre input/output on those speakers, because AFAIK there's no such thing as 3.1 speakers anyway. I can't set my Audigy 2 to output 3.1 for instance.
I dunno whether the Aego M upmix from 2.1 to 3.1 or need a centre input. If they don't upmix you would have to set your soundcard to 5.1 to get anything coming out of the centre, but you wouldn't hear rear sounds at all. You would have to buy seperate powered rear speakers.
You really need an Aego M owner in here to tell you. I'm sure they sound fine just as a 2.1 set though without a centre.