I wanna get one sub because they r expensive and I plan to compete with it so I still need to get the big 3 upgrade up my alt. To 250 amps and get some deep cell batts. And get sound deadner so that's why I'm going with one for now than later get another with a nother amp. how much of a difference will 18 inch fi btl n3 sub be to a 15 inch sub I do plan to get another one of whatever I decide. So what will be better