I'm not familiar with the technical side Infinity subs although I have heard a few. A good sub makes a huge difference in your audio experience so enjoy.
Regarding the crossover dial on the back of the sub. No, the receiver has no idea what you are setting it to. Things can get out of whack if you try to use both crossover settings so I suggest that you set the crossover dial on your sub to "Off" or whatever setting means that it won't do any crossover function at all and then let the receiver do the job.
What type of Axioms do you have for your front mains? 80 HZ is probably a safe bet to set the X-over for just about any set of Axioms.
Another hint. Subs (good subs) work in frequencies that are pretty much non-directional. So from that point of view, it's not just blending in with your left speaker but it's blending in with the whole room.
(If you walked into a strange room blindfolded, you should be able to hear the lows from a subwoofer but not be able to point out what direction it's coming from.)
On that basis, you do not need to worry about getting a matching one for the right side. HOWEVER, you may be able to improve the sound a lot by experimenting with it's location and adding a second sub is often a good idea when you can afford it to help get rid of any areas where the room is canceling out some frequencies.
So yes, long for that second sub and by all means, play with the positions. But take comfort that you don't need one on both sides just because you have a main speaker on both sides.
Any deep bass that draws your eyes to the left is most likely your mind getting tricked by mid-low bass coming from your left front speaker.
