If you’ve only one pre-out, just use a simple solid rca splitter. Run to one of the inputs on the miniDSP, then run 4 outputs, one to each sub. Bypass the XO on all 4 subs, if you don’t have a bypass then just wind the frequency pot to the highest setting. Set your phase toggles to zero.
Then set up your crossover points in the miniDSP. You can now do some basic time alignment and EQ, though it’ll be trial and error without a mic. If you like what’s happening and you want to go a stage further, spend another 50 buckazoids on a USB Mic.
The miniDSP non HD can be had for about $100, so without a mic that’s your total outlay.
Incidentally, I did NOT have the same experience as MillerCarbon, adding 4 subs without a measurement device and EQ unit to create filters and add a room curve. It may be down to the shape/size of my room.
Four subs no room measurement blah..
Four subs plus miniDSP whoopee
Four passive subs, two high powered digital power amps and miniDSP WOWZEE.