You're shopping for your own Christmas present?
That's cool- at least you won't get more ties!
Those JBLs you have aren't too shabby. If you like the general way they're voiced but just want to kick up the detail a notch, you might want to look at the Monitor Audio Bronze B's. Similar voicing but just less veiled. I'd suggest B4's and a Bronze B Center if you can get them for what you have budgeted. Keep your current speakers for surrounds.
The Athena's aren't bad for the money, either. But I don't think they're really any better than the HLS-615's you already have. Those JBLs are pretty competitive with most current speakers around $500-600, IMO. A Bronze center from the original (pre-Bronze B) series is a reasonable match for them. You could try looking for one of those. I mention this because I'm listening to my HLS-610's hooked up to my PC rig right now, and I have some Monitor Audio Bronzes in my bedroom stereo rig 8 feet away from me. A few years ago I tried mixing and matching Bronzes & my JBLs and was suprised that they matched reasonably well. The MA's & JBL's both creamed a pair of Infinitor Monitor series speakers my brother left with me.
I'm a big believer in using matched speakers, at the very least for the front 3 channels. I haven't heard the Axioms so I can't really comment there.
Alternatively, you might stick with your current speakers for awhile and get a new sub. That's probably the weakest link among the ones you listed. A new Hsu sub would be in your price range, and they make some nice ones. Def Tech makes some you could afford, too. I'd hate to recommend one without listening, but SVS has a good enough reputation that I'd feel comfortable doing so. Plus they have an excellent return policy if you don't like them.
Just a few ideas.