Your specs should tell you at what loads it is stable at ie 4 OHM steroe, 2OHM mono, or 4OHM stereo, 2OHM stereo, 4OHM mono.....it all depends what it is rated at....if it is rated for 2OHM mono then you can run 2 4OHM subs bridged to get a 2OHM load, but if it is rated for 4OHM mono then you will need to get 2 8OHM subs to bridge it mono....what kind of amp is it, that will help out alot.....