I own a pair of JBLs Control 1's, they're not as good as the reviews would have you believe. They only go down to 70 Hz and that leaves a definite lack of bass, plus they sound a bit 'plasticy'. I've never heard Kef Cresta 10s btw, but Kef have a really good reputation and since they should retail at over £100, getting them from RS for £60 - you can't lose really.
I expect they would produce better mids/highs than the Aego Ms but they won't produce the deepest bass notes the Aego Ms sub is putting out. The bass will be perfectly fine for music, but maybe a bit lacking for the really deep rumbles in games/films.
Seems a lot to spend (on amp + speakers) if you already have the Aego Ms, just for the convenience of a headphone socket. Can you get a front drive bay for your soundcard to give you a headphone output? You could even get a 3.5 mm splitter cable, although I've read they reduce quality. Maybe your case has a headphone socket? (they connect up to some of the pins on the soundcard)