DSQ, you may have ordered already, and either choice should easily drive your Axioms with no problem to beyond safe(to your hearing)listening levels. It can be noted, however, that the 4520 certainly doesn't have an "official" 4 ohm rating. That can only be had in conformity with the FTC amplifier regulation procedures, which among other factors would require it to be driven at full rated power into a 4 ohm load for at least five continuous minutes(an unrealistically severe burden outside the test lab). It carries 8 and 6 ohm ratings.
The difference in the maximum power ratings(150 vs 125 watts)is trivial and amounts to only a 0.79dB difference in maximum sound level.
You're wise to consider an A4L refurb and either one would have all the key features, including the in my view essential Audyssey XT32. If as you say you'd go with the Marantz if you had no amplifier worries, then do so, because there're no solid grounds for any such concern. Enjoy.
