Guys.... please pay attention to the details. He's looking for a Class A/B amp (for some silly reason). And he's looking for 750w. And he's got a Single 4 ohm subwoofer. So your listed amps need to do anywhere from 500-1,000W of Class A/B power at 4 ohms to meet his requirement.I hope you've got some beefed up electrical for that btw.
What subwoofer do you have?
I'm sure you already know this, but I just can't sleep without saying it. You could easily get some very fine Class D mono amplifiers that sound just as excellent on a subwoofer as whatever class A/B you find, and likely much cheaper.