What semi said x2.
Also.
Is your current sub in the optimum enclosure? Getting the most out of what you have is the best option to start with. Most 15" subs need large enclosures, two need twice as much. How much space do you have available? How big is your current enclosure? Is it sealed or ported? If its ported, what frequency is it tuned to?
That pioneer amp is not really suited either. Only 250wrms at 4ohms, not 1ohm stable. Should be ok for daily listening, but if you push it to hard it may clip. This is bad for the sub, it can let the smoke out!
I'd make sure the enclosure is the best i can make it, before anything else.