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.
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Look at the 6.5" T Zero and the 8" T5 REL Acoustics subs.
The SVS SB 1000 is a great match with monitors. They have a 45 day in home trial and free shipping! I have used it with Harbeth and Spendor and it blends perfect with them.
Used RELs are around if you're patient…I use both a 10" 150e and an 8" 108e mkII, and I imagine either would do the job for your system as they're amazingly well designed. I paid around 200 bucks for each and they were in excellent shape.
Check this one out. It is a discontinued model with full warranty from SVS!
thanks everyone.  I ended up picking up a svs, one of the sealed models.  Looking forward to getting it. 
Good luck, Sam.  Running a pair of SVS SB1000s myself.  Couldn't be happier.  Don't have experience with bookshelf/monitors so I didn't chime in earlier.  With floor standers running down to (80-50?) Hz on their own before things start dropping off noticeably, these SVS subs blend in very well driven with speaker level input.  I was surprised the improved sound isn't just about the bass.  There seems to be more air and body at frequencies above the crossover point.