Kinergetics made a pair of subs specifically for this application, as well as stands to hold both them and the Spicas, and amps/crossovers to drive the subs. It was popular in some very high-end audio dealerships 20+ years ago.
I heard many people criticize it, but personally, I absolutely LOVED the package. Not sure why I didn't actually go through with the purchase myself at the time, but I think I was chasing a pair of Legacys - which after finally hearing them at the same dealership, felt they absolutely paled in comparison to the Spica/Kinergetics.