I think that Denon should be more than sufficient. A separate amp certainly wouldn't hurt, but is by no means necessary with either speaker. The JBL's seem pretty effecient with an 8ohm/91db rating. The 4306 is pretty beefy, so it should be fine. The Axioms seem even easier to drive. It will come down to which speaker you like better as both should be well handled by your receiver.
If it were me, I'd at least give the Axioms a shot because you get to try them in your home. I don't know who you're getting the JBL's from, but maybe you could swing having them let you bring them home as well where you could do a side-by-side comparison. If it's worth a sale to them, you should be able to make that happen. If you don't like the Axioms, at least you know you've judged them in your environment and all you're out is shipping (back) and a trip to ups/fedex/usps.
Also, I'm certainly not a fan of judging a speaker by it's posted specs, but it doesn't seem like you're getting a lot for your money with those JBL's...the posted specs show a freq. response of 80hz-22kz at -6db (not the usual or preferred +/- 3 db). Must be their enclosure (if I read correctly, it's only 6" deep), but the cutoff being at 80hz with 8" drivers seems odd to me. You'd definitely need a sub. Just my opinion. Good luck!
