The Fathom F113 was much better SQ wise than any other commercial subwoofer I have ever owned, only beat by the DIY TC Sounds LMS Ultra sealed. The F113 bested the Revel Sub15, Vandersteen 2Wq, Bag End Infrasub 18, Various HSU and SVS subs, Velodyne HGS-18 with SMS-1 and about 20 other well known commercial subs. What differentiates the Fathom from every other sub is the pitch articulation and accuracy which makes you hear things in bass lines you have never heard before, everything else sounds like mush in comparison. The downside with the F113 is a 13.5" woofer can only move so much air and as you lessen distortion the perceived volume sounds lower so it is best used in multiples.
I do have a friend who has a pair of Seaton Submersives and those are excellent subs and would be my second choice to the F113 as far as SQ is concerned. The Submersive kills the F113 in shear output though but that should be expected when comparing a single 13.5" driver to a pair of 15".
Like others have suggested the ultimate would be DIY and going with the TC Sounds LMS Ultra 18" driver which can be used in an enclosure as small as 3cuft sealed. The only sub that matches the SQ of the F113 or maybe even exceeds it but can also play to much higher levels and go a lot lower.