What else would you be using the amp to power? If the answer is nothing, then don't bother with it, just go straight to the mono...
Normally 2 channel amps have one of those channels made for a sub, with crossovers and the likes... That second one is set up for that, but you will only get 250W Max out of that (which is piss all)...
When looking at amps, never read the MAX power reading, its bullshit... a marketing gimmick at best... read the WRMS reading (Watts Root Mean Square). This is a test all amps have to do to calculate how much power they can put out for a specific amount of time, SAFELY... as opposed to the MAX reading, where they pump 16V into it and burp it til it dies...