DSP in bass is interesting. One of the single best things you can do using rephase is to unroll the phase issues caused by the reflex port (assuming you have one). This really tightens up reflex bass, retaining the benefits of the port and removing the drawbacks.
Phase oddities generally stop your crossover working correctly, so improving the phase of your bass is a big step to getting it all working really well. I imagine that a lot of the issues people have in making subs work.
Oh, all this is only possible if you use a decent computer for DSP. A Najda or miniDSP won’t cut it, I think.
I have the Mini DSP 2x4 HD, it is more than adequate, allowing full independent control of a pair of active subs and a pair of speakers so they can be integrated together.
As someone who is a digital philistine, I seem to have overlooked the massive elephant in the room. Analogue into digital (DSP trickery) digital into analogue,
but if you can find out how many taps they support you can just see how close rephase will get.