My advice is to not buy something that you'll have to sell at a loss later. Perhaps if you want two subs in the future, just buy one now, and the amp for it. For example, you could buy an Icon and a sundown SAZ1000D or something, then when you get additional funds you can add another Icon and another 1000D and daisy chain them. Or if you want FI BLs, maybe you could buy one now and a small 2-channel amp to fit under your budget. Then when you get more funds, buy another BL, buy a bigger amp, and switch the smaller 2 channel over to powering your front speakers.