I have purchased two of the twin stacks to mate with the M3's. I have not even been able to start the build yet, (3 months) as time has not permitted. I am currently using an old Klipsch 10 inch sub that is doing "OK". Really looking forward to getting them built by Christmas ! That was just one of my little let downs with the M3's (for me), they are a bit thin on the bottom for my liking. I could get down to 38hz in room. They may get lower "in room" if I moved them around, problem is I have them set finally where everything from mid bass to the top sounds very good. I use Dirac-Live to clean up the bottom end up to the 200hz mark. Once I build the subs I will still use Dirac-Live to help with integration as it does take a lot of modal energy out of the room.