The best thing to do, use that amp for the doors or bridge it for the sub, either way you need another amp. running it mono would'nt be a good way to set it up, not saying that you can't but for the purpose of a crossover and just having stereo sound it would be better. Bass is usally mono, above 150-200 hz usally is, or if you think it might be a problem just run the right and left channels together and it make it mono.