used 15" IDQ we need to know more info to help you out. How many subs what size how much room we are working with. How much power you want to send. SPL or SQ there are too many variables to decifer through.
He might want to upgrade the amp... he should put about double what that amp is putting out to each woofer, what's it doing? 300 a woofer?... about 600 would get them to wang... what size box are they in?? Sealed or ported? I had 2 12w3s with 500 watts a piece and they sounded awesome and were loud as fuq. They were sealed with probably 1.2 cuft each.