It looks like you have impressive research experience, which I think will really help you! I do not think a 154 will keep you out of all of those programs, especially Northwestern.. I'd encourage you to check out their GRE stats (they publish them on the website). With a lower quant score, you really want other parts of the application to be top-notch. From everything I've been told, the statement of purpose is very, very important. If you haven't already, you should ask your recommenders to take a look and give you feedback. Personally, I would not apply to Amherst or JH.