I am using an inexpensive dbx analog crossover in my system. I toyed with the idea of using a mininDSP and will get one someday, but decided it was too much hassle just for a crossover. If I was designing a speaker system or using a multiway crossover and optimizing time and frequency response I'd get it for a crossover, but just to emulate an analog crossover seems like overkill.
I have not used the Rythmik HPF, IMO probably not worth the effort if you are going to use the same crossover. The miniDSP has decent opamps but you have to get the balanced version to have enough drive for most applications. You are also adding another A/D-D/A conversion in the process so it is more than just op-amps. If you need it to generate room compensation EQ or need more flexible filters it is great, but if all you need is a HPF and are not changing slope or crossover frequency, chances are the Rythmik is good enough. Brian talked about the built-in HPF someplace in this thread.
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley