Hi Starbuck,
Welcome. I think bigjohn's got your situation covered pretty well.
I'd encourage you to peruse these forums some; there is quite a bit of collective opinion to be read. I was going to say "collective wisdom" but that phrase somehow enobles the regulars beyond merit
Definitely see if a friendly Axiom owner lives nearby; many folks are amenable to auditions. Make sure you stop by the Beer Thread. Stay away from the Politics thread
You are going to realize a significant improvement over your Bose system with any Axiom setup.
I have a couple of concerns/observations about your situation.
First, you are used to a pretty big, diffuse soundstage from the 901's. While the clarity and precision of Axiom speakers is going to be "better", it might take a while to adjust to the change. Also, I wonder whether an Axiom floorstanding model might more closely match the fullness that you've been listening to. If you opt for a bookshelf model, make sure to budget for stands.
Second, your listening habits give me pause. Axioms are EXTREMELY accurate speakers, and recordings from many hard rock or metal bands are not mastered very well. You may find that Axioms reveal undesirable traits in those recordings. Some forum posters have found that the sound of the M50 is more to their liking than the M22 or M60 (which have similar sonic signatures). Forum regulars kcarlile and 500 are M50 devotees, and may be a useful resources.
Finally, give us more detail about your Yamaha. You may not be as bad off as you think there. The speakers are going to make a huge difference; I'd urge you not to scrimp on speakers unless you don't have any Dolby Digital processing whatsover. Even then, you can find good pieces from HK and Denon in the sub $300 range.
