What's the rationale behind your interest in the Fathom subs? The 112's house woofers of the same diameter compared to the ML's, but the former is no doubt more powerful, both driver- and amp-wise. The question is what it amounts to in your context apart from more headroom/sense of power perhaps, but the DSP section of the Fathom's (that has been pointed to already, and which is more sophisticated), may be the most determining factor for potentially better room integration. In addition I'd argue that subs-main speaker integration, as well as overall main speaker potential, would be further enhanced high-passing those Sabrina's, but that involves adding a digital processing layer via a DSP (unless done passively), which may invite controversy either for simply being handled in the digital domain, and/or that it's a second processing layer on top of the passive crossover prefitted in the Sabrina's.
I share the sentiment expressed about the miniDSP route for less money and same-ish results, but if money isn't an issue (and it seems it isn't, certainly up to about $10k) I'd up the ante and challenge even further for a separate amp + DIY subs solution of a considerably larger and more efficient kind. There are many options of such a large, high eff. and separated sub+amp+DSP (+ high-passing the mains) route, and I'll refrain from elaborating any further here being it's likely an option most will choose not to embark on, but the sonic perspectives offered with such a solution is one that really separates the sheep from the goats; those small all-in-one subwoofer cubes, expensive they can be, and added to full-range running main speakers are really only the tip of the iceberg performance-wise, but who am I to argue against a convenience-catering industry and its followers.
In any case I'd try and make some sense of the madness. From all I can assess the inclusion of the Fathom's will likely be more of a sideways step to your ML's than a ballistic upgrade, so why not spend your money on a turntable and more music?
