I have two 12" subs designed specifically for music by Rythmik and GR Research. In a very resolving system with full range speakers (mine play down to 26Hz before gently rolling off) you will find that some recordings sound much better with the subs and some sound worse. It's because of how different each recording emits bass. Most of my 60's and 70's rock recordings are deficient in bass and love added sub woofer with the crossovers turned fairly high. But the occasional excellent production/mastering/mixing/quality media recording suffer from the added bass. If a person were willing to dial in the sub for each recording it could work great but I find it best just to turn them off for really good recordings. Otherwise they just blur detail. All of this assumes the subs are setup correctly.
Bottom line, I think your stand mounts with limited low end would love a sub but a full range speaker with highly resolving system can be hit and miss. Still worth having though. Oh, and I would recommend considering Rhythmic with the custom audio grade driver by GR Research. They use servo technology unique from other servo units and sell direct so the bang for buck is serious.