multiple subs

I am wondering what everyone's opinion is on whether it would be easier to get a loud system using several cheap 15-18in subwoofers vs something like 2 nice 15s or 18s. I have a friend that just wants a really loud system and doesn't care about SQ but doesn't want to spend too much money. Ive...

What's better 2 15s or 4 10 inch subs? I'm really limited. Only space only about 5 cubes. Not really enough for 15s I think unless sealed. But 4 tens I could go vented or ported. I have two 15s right now. But will 4 QUALITY tens play louder and lower than the same QUALITY 15s.