Actually RPM is an idiot, the Type R would be a decent choice. But now looking for a cheap but good sub for a 15" box, good luck. I will give you a link to check out for anything that your looking for sub wise. They are low cost, very good subs (pound hard) low cost because you buy directly from the manufacturer, comes with a 5 year warranty and kick a$$ customer service.
http://www.edesignaudio.com/index.php?cPath=1_21