I am considering a miniDSP SHD Studio, all digital with no converting to analog, and then sending out to 2 DACs (OKTO DAC 8 to main speakers / Schiit Modius to my 2 15" passive subs). Amplifiers are Benchmark AHB2 for mains and a Crown Class D (750 WPC) to the subs. All on AES/EBU and balanced.
Minidsp should do the trick. Their QC is a lil poor. Check if everything is working and get a replacement unit otherwise. If you have pre-existing 15 inch subs for intended use, you may be restricted to crossover no higher than 100ish hz. The specific rythmik sub I was talking about earlier can also serve as a mid bass unit and get down to 20 hz.
Depending on your room and freedom of placement, you could try your large subs at the 1/4, 1/4 and 1/4, 3/4 widthwise, lengthwise locations. With adjustable phase, you should be able to get rid of a lot of problematic room modes and hopefully still not localize anything. If you have 2 subs or more, a read of the Harman paper outlining the virtual sub placement principle could come in handy (as you play with placement).