subwoofer to match with my bookshelf/monitors

I have dynaudio x-14, with a NAD c356bee integrated amp, and marantz cd player.  I love the sound quality of the system but wish it had a little more bottom end.  Are there any powered subwoofers in the 300-700 price range that would be well suited to match my speakers I dont want them to over power it, would like it to be well balanced, I listen to all types of music, rock,blues, rap, pop, booming bass is not important would like clean crisp bass.  I have been looking at the svs sb 1000 reg price 500, and monitor audio bronze w10 on sale at 500.  but cant listen to either.  Other models, brands, what size driver power size should I be looking at to match my system.  Any help would be great.
thanks
sam