Modified Adcom 5400 or Stock 5500?

Art's comments spurred me to add that I have owned both the 5500 and 5503 amps, and gave both of them to my son when I upgraded by own system. My son previously had the Adcom 545, which has about the same power output as the 5400, and when it was replaced by the 5500, there was a significant improvement in the sound of the system.

All other things being equal, I'd go with the 5500. The Maggies usually perform better with more power, and I think the general sound quality of the 5500 is better to start with than the unmodified 5400. You can always have the 5500 modified at a later date to improve the sound quality, while it would be impractically expensive to have the 5400 modified to equal the 5500's power output.