If that amp is rated at 1000 watts with a 18hz hipass, modifying it to 8hz with a sealed 18" Mal will drop it performence level quite a bit.I would look at some pro amps like the QSC 1450 or 1850HD.They can be had used on ebay cheaper.I've never clipped mine and they do get a work out from time to time.
Real world listening is much easier on amps to, to play Devils advocate. You can watch WOTW in dts and it will never compare to a 10hz sine for two minutes !!!!
Stick with pro amps , you'll never be sorry.