@Ormy yes you're right, whenever you discover something that make either music, home cinema or both sound better it is a bit of a thrill!
If you get a brand new minidsp, plug one subwoofer input to it, connect four subwoofer outputs but don't do anything with a computer connection, does it default to just being a signal splitter/booster? I.e. no EQ or anything just sending the signal input to the 4 outputs?
Not quite. Default for the 2x4 is to act as a stereo active crossover. So input one is routed to output one and two with a low/high pass accordingly, input two is routed to output three and four in the same way.
You don't have to have the minidsp connected to the subs to be able to configure it though. So you could plug it in to the PC, connect, and then set it up in the way you've described. Then disconnect it, plug all the subs in, and leave it.