Subwoofer to match Vienna Acoustics Bach

I recently purchased a pair of Vienna Bachs. Currently I have a Harman Kardon amp/tuner..I listen to a variety of music, but also use this setup for home theatre. I am wondering what type / model subwoofer would be a good fit with the Bachs to compensate for the low frequency effects needed for many movies (in the $500-1000 price range).

Any ideas would be much appreciated.