you can connect as many subs as you want to a mono amp - you just have to wire them correctly to get the correct load on the amp.
The sx1250 runs optimally at 1 ohm, where it produces 1250 under-rated watts. There are several subwoofer configurations that allow a 1 ohm load... A pair of dvc 4's - each sub gets 600w+ 4 svc 4's - each sub gets 300w + One dvc 2 - Sub gets 1250w
Also you COULD run the amp at 2 ohms where it produces 625w. For that you'd need... A pair of svc 4's - each sub gets 300w+ One dvc 4 - sub gets 625w A pair of dvc 2's - each sub gets 300w+