The REL ST series of subs that use the AB class amps are the ones to seek out IMHO. Try a Stadium lll with the amp/crossover removed from the enclosure and use the exact same speaker cable as the main speakers use and be prepared for an absolute revelatory experience.
I use MIT Shotgun S3.1 biwire speaker cable from my amp to speakers and use the same from the Stadium lll amp to the sub and a rear placed Strata lll sub for absolutely seemless integration and bottomless extension. One cannot tell the subs are even in operation.
I do have full ASC tube traps in the room along with golden Sound acoustic disc corner treatment. The soundstage is huge when the recording is such and deep and tight when otherwise. Yes, unconventional but oh so worth it!