Problem 1... bass boost...
Bass boost is bad, mmkay! For every 3dB of boost, you are forcing the amp to try to double it's output at that frequency. Not good! Not only does it give you a hugely bloated response at the boost frequency, it sucks the powersupply clean, and produces clipped, noisy output. Turn bass boost OFF and keep it there.
Your HP frequency is looking pretty high. You want to set that to 10Hz below the tuning frequency of your sub box. If you don't know what that is, just set it very low.
Now play around with the LP frequency to get it to blend well with the front stage. Ideally you will want ~80Hz or so, to blend in with your front stage. Make sure that voices don't become boomy, otherwise it's set too high.
Finally, your gain adjustment. Read this FAQ to learn how:
http://phoenixphorum.com/gain-setting-w ... vt280.html
Basically, you will want a 50Hz test tone. Disconnect the sub, and connect a multimeter to the speaker wires. Now, since you want 1500 watts at 2 ohms, we use the formula sqrt (1500 * 2) = 55 volts. Adjust the gains until you reach this point. You can turn it up above this if you've got a remote sub volume control knob, but you must realize that if you play tracks with loud bass, you have to turn the knob down until it is not clipping anymore.