XLS400 is a very good sub for the money
Fast and musical and will dig deep when necessary - basically at about 1/3 power is enough for my semi large for an average house listening room.
As with all subs its getting them setup correctly that will make our break your listening experience.
Also I would strongly consider a DSPeaker antimode unless you have proper bass trapping in the room - otherwise its very likely to be a mode fest with any sub.
They have new models out - the cinema model which is for one sub only and has more filters than the older model so will be slightly better - for similar money