Limiting a full range driver

OP:  Sounds like a good idea, but, this is key, be ready to measure your results.  Once a speaker/subwoofer gets into a room throw the specs out the window and pretend you know nothing about the performance of the speaker, because without measurements you really don't.  The room changes everything so while I think your approach to using digital miniDSPs is sound I also think you'll need to carefully gauge the setting and for that you'll need a mic and software.

It's an 8" driver and it can handle but low frequencies without distress.  If you really want to set a low limit you should aim for a 60Hz or less.  Do you really want your 15" subs handling the upper bass region?

I plan on getting the UMIK-2 (the better one) because the miniDSP also has Dirac that will need measurements. I know the 8" can handle the lows, but does that prohibit the midrange from sounding its best?

Unless you have larger and very good quality 20-20000 hz full range speakers, offloading the lowest octaves to a proper sub or 2 designed specifically for the task at hand is pretty much always a good move. You just have to choose the right sub(s) for the specific job at hand and do the integration right, which takes a little know-how. Definitely measure before and after in order to do the job right.

miniDSP and UMIK 2 mike is a very good choice. I use that mic with Room EQ Wizard to apply room correction in Roon DSP similarly both in one room with sub and several without. miniDSP is on my list as a potential future experiment in tweaking my sound both optimally and cost effectively.

I do observe that on paper those REF3a’s alone do look pretty full-range capable, depending on room size, with the right amp to enable that. Even then offloading bass to subs properly will allow the amp to not work as hard, theoretically lowering distortion, and that combined with the right powered sub or subs in the mix should yield clearly positive results, if done right.

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).