My opinion (not experience
) is that the Denon 4306 is a pretty high-quality piece of equipment that does likely produce close to its rated power with very little distortion.
The Crown amps (and some other "pro", instrument or PA amps) have higher wattage ratings, but can sometimes run loudly (apparently the QSC and Carvin are not in this category). Generally, they also tend to generate somewhat higher rates of distortion. I was unaware of QSC, but am now quite fascinated with their products.
There is a decent short article here
that talks about the importance of not only power, but also distortion and signal-to-noise ratio. Axioms are already very efficient speakers. Remember that to get a volume twice as loud, you need TEN times as much power. If you do not already own a SPL meter, I strongly urge you to get one (like the analog Radio Shack version) and quantify just how loud "loud" is to you. Listening for long periods above 110db can cause permanent hearing damage, and I suspect that your Denon is capable of generating 100db with your speakers without too much sweat unless you are listening in a cathedral.
To echo The Rat, it definitely depends. If you listen at very high volumes AND in a very large room, the extra headroom might be useful to you. I do not assert that employing separate amplifiers makes no difference. However, I continue to believe that it is also important to ascertain whether or not you REALLY can discern a difference, whether the difference is preferable, and whether you find that the investment required justifies the incremental difference.