Hi PD,
Well, I do hope the Ultra makes you as happy as it did me. I've never heard a Fathom, so you have that to reference, but the Ultra can get the job done. It is a great sub woofer. And as you have seen, the Ultra out peforms the F113 as you go down in frequency. I think you'll prefer at least dual Ultras as Ed said, since your room is large.