I use two small sealed subs in my setup, equalized with the Velodyne SMS1. Both subs are situated between the speakers. I love the results. Lot's of technical info in prior posts, but one key for me was to cross over the subs to kick in BELOW the lowest frequency of my monitors, so that the subs do not overlap the monitors. Trying to integrate subs to overlap with your monitors, or to otherwise replace the low frequencies of the monitors, is a different beast and very difficult to accomplish in most rooms, in my experience. The other key, was to learn to set the subs at a modest volume level. It's tempting to crank the subs volume, but as is often said, if you can hear the subs they'rep probably too loud. This is especially true in a music only system. I can't hear my subs specifically, but if I turn them off, I immediately miss them. It took a lot of tweaking to get my subs to integrate the way I wanted, but it was worth the effort. I highly recommend the use of an equalizer or equalizing software and a good spl meter is a must. I also recommend a sealed sub - I had a ported sub previously and it was much more difficult to control in my room (YMMV).
