Erik, you're not going to be "giving my Axioms much more power" unless you turn the volume up higher, possibly endangering your hearing. The all-channels driven spec is meaningless in real-world use at home, as distinguished from laboratory stress tests. You can be confident that your 7400 is capable of at least 105 watts of output(probably a bit more, since receiver power ratings are typically slightly conservative)if on a rare occasion a brief peak in one or two channels actually called for it. Doubling the maximum power output capability to 210 watts would simply mean that the speakers could play a brief peak 3dB more loudly(e.g., 108dB vs 105dB)and would be meaningless unless you actually turned it up that loud. Unused headroom is simply that: unused. So, you won't hear "the" difference because it's unlikely that there'll be "a" difference.
Getting a new receiver for more up-to-date features(e.g., Audyssey MultEQ room equalization)can be a different matter, but don't waste the money that you saved on the sub purchase by chasing an essentially meaningless power spec.
