Sub suggestions

My brother uses the LSA statement integrated with a pair of Martin Logan Summits. He uses a REL and loves it. I also use REL with my Dynaudio Contour 3.0's. The REL hooks up off the MAIN amps output so it gets the same signal as the main speakers this gives you seem less integration. A used REL B2 or B1 would be great. Some of the new RELS are great also.

I'll second the SVS SB 1000s. I bought a pair delivered for under 1k, and they're tremendous. Great company, great product. Highly recommended.

I agree that the SVS looks like a great value.

One thought FWIW:

I once owned deCapos (not BE). To my ear, they were a bit rolled off on top and unusually extended in the bass (for a stand mount). The result was a well-balanced, extended (rather than full) range speaker. IMO, they weren't a great candidate for subwoofers because they were that unusual small speaker designed for neutral response and balanced accordingly at the frequency extremes. I'd think the top end would go a bit dull if you use a sub to enhance the bottom end.

I'm not sure at all that this would apply to the BE, but I personally wouldn't use a sub with the original deCapo.

The sub I have now - the Velodyne 8" CHT - adds a bit of umph! and some bass depth; I have the volume set pretty low and the crossover set higher than it should. Still, it comes across as muddy and slow at times. The BE tweeters do brighten things up more than the original tweeters did, so I'll be happy with a faster, more smooth sub.