Tide fan, the 3900, along with all other competently designed modern receivers, has a flat frequency response over the entire audible 20Hz-20KHz range. There's no way that it can cause a "brightness" or any other sonic characteristic that isn't part of the program material that's being played. So, what you read isn't true now and as far as is known was never true. If you want the more up-to-date features of the 3900 it'll work fine with your Axioms.

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Enjoy the music, not the equipment.