I just got a pair of Summits last week, but my initial impressions including the bass are - Wow!
In regard to down firing sub surfaces, my initial thoughts were that the Summits might require a hard surface under them instead of carpet from past speaker experiences. I had a down firing sub woofer and when MDF was placed under it, it seemed more articulate and somewhat tighter. Also, before my Summits and SL3s, I had Carver Amazing IIIs which also had a down firing sub BUT employed an MDF base as a surface for bass reflection. Both examples lead me to believe the Summits would require the same but I do not believe so after hrs of play time and adjusting the 25 and 50HZ settings per speaker.