Hard to tell since we don't have T/S parameters for the SP15, but the way the box is now, it's tuned to around 62hz and the Fs for the SP15 is 25Hz so your slightly over damped. If it dosen't sound boomy I'd just leave it alone. Other wise just build a bunch of 10x4 frames about 1/4 to 1/2" thick and keep adding them in there to lower the Fb of the cabinet. HTH Bill W.