@cooper52 --
I use subs in my setup that "meet up" with high-passed main speakers, but they’re all outboard actively configured via the same DSP/digital crossover, mains + subs, and thus high-passing the mains isn’t an extra measure per se. Rather it’s treating the mains + subs as a single 4-way speaker system per channel as an outset with a DSP unit that handles crossover duties for the entire speaker system (save for the "super tweeters" that are passively configured), and so the high-pass filter over the woofer/mids in the mains is just a natural measure on top of the other crossover points to have it all mesh nicely together with the separately housed subs.
While I’d typically recommend high-passing the mains in conjunction with the use of subs, in your case with passively configured main speakers - and depending on how much you’re willing to invest, one way or the other - I might go with simply adding on a pair of active subs (not just one) and experiment with their built-in low-pass setting to best gel with the mains. Don’t be afraid to have the subs overlap a bit with the mains (this can be beneficial), and also try and find the best slope and not least phase setting. With regard to subs placement, as an outset I’d set them up flanking the mains symmetrically, one sub close to each main speaker. Or sidewall placement, again symmetrically. To me symmetry to the mains placement of subs is more important than diagonally or otherwise asymmetrically trying to optimize smoothness of frequency response, even when low-passing the subs below 80Hz, but to each their own. If you would do so anyway, try out a DBA with at least 3 subs. Experiment and see what you like best, and maybe you already do know.
I haven’t used miniDSP iterations myself, but know people who do and have heard their active setups. From what I’ve been able to assess though the miniDSP doesn’t cut it for me, and so my recommendation would be for very high quality pro DSP units like ACX (previously named Xilica), XTA, LabGruppen or - even more expensive - DEQX. All of these are great, transparent digital crossovers, and somewhat more costly than the miniDSP’s.
But again, that’s just me in the context of a fully active setup where I need an excellent DSP to handle crossover duties over the entire frequency spectrum of mains + subs. Nonetheless, in your case I’d either go all-in with a quality DSP unit to high-pass your mains and optionally low-pass passive (i.e.: sans both internal amps and XO’s) subs with outboard amps of your own choosing, OR forget about high-passing your mains, run them full-range and add on a pair of active subs with their built-in low-pass filter.