Bass management--high/low pass filtering

A minidsp SHD (not the best quality perhaps, but your mileage may vary) can be used as a streamer with bass management, if you intend to keep the modwright in the chain. They also have other models in their product list.

For a more polished/ turnkey 1 box solution, the Yamaha R-N2000A amp (w/ integrated bass management features) will do it really well. I got to measure this unit.

Accuphase used to make some very high quality units

etc

Guys like @phusis may have suggestions on other items that can be used to achieve the above.

@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.  

Once again, this has been quite the education. Thanks to all for your suggestions and input on the subject. In the end, this seems like too many hoops to jump through to get the high-pass filtering to work in my system, and my previous experience with a sub (my late, lamented REL S/2) tells me that, as interesting as it would be to try, it's just not going to be practical. In particular, I'm not willing to invest $3K plus in JL Audio's CR-1 just to see what would happen. 

My room and my setup preclude using a pair of subs. I can only fit one in that space, and the REL sub filled things in admirably. For our listening tastes, at the volumes we are comfortable with, one sub is more than adequate. 

Still, I've learned quite a bit from all your responses, so again, thanks!

Every room, and every system is different. My take is don't buy new stuff until you need it. Also what is the problem you are trying to solve for? Like are you missing bass, or are you missing deep bass?

Run the mains full spread, add the sub, adjust it to fit your mains. Then live with it for a few weeks, it will take several adjustments to get right. From there, figure out if you want to cross over your main speakers or not. 

My HT has everything crossed over, my mains are at 60hz, subs are also at 60hz, it works well there. Plenty of bass, clean sound.

The 2ch has the mains running full, sub crossed at 40hz. Tried all the hi-pass and low-pass settings. It was just missing something. Lost body and tone on my mains. Sounded almost hollow in spots. After weeks of messing with it, went no crossover on the mains, and 40hz on the sub, the light just clicked, this is correct. 

My other half felt the same way. Was listing, she walked in, looked at me, and said "this sounds great". That was all I need to hear about my settings.