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.