If you haven't already there are a couple things you could try that might help your bass response besides a new subwoofer and/or amp.
Try repositioning your speakers and/or your seating. Near wall and corner placement can play havoc with your bass as can certain combinations of speaker and seating locations. Here is just one article there are many more that you might find more helpful for your situation.http://www.gcaudio.com/resources/how...placement.html
Even if you can't move and leave your speakers and/or seating in new positions you should at least try experimenting some. It's very possible that your room's acoustics are playing a big role in how you are perceiving the bass. If you find that the bass improves by moving your speakers or seats around then this may help solve our problem.
Even if you can't leave things in the new positions you could now look at adding some bass traps to help balance out the bass in your room. There are some good DIY articles and threads out there on making your own bass traps.
As you've already figured out a sub could help by allowing you to place it for best bass response while still leaving your mains where soundstage or aesthetics require you place them.
I'm not a big fan of amps making a huge difference in speaker sound but bass response is one place I've heard a difference. If you know someone with a separate amp you can borrow you might want to try it out before spending a bunch of money. Or at least make sure to get one with the option to return it.
However, before spending any money I think your best bet is to play with your speakers/seating positions to determine what if any effect your room is having on what you're hearing. I say this because, of the complaints I've heard about Axiom M80s and M60s lacking tight bass is not one of them.