Noisy files when converted

I'm downloading hi-res FLAC files from places like Linn & HDTracks. Many of them are 96/24. However, when I convert them in either XLD, Max or Switch to play on my Mac (AIFF), many or the tracks have a horrible hissing noise that ruins the listening experience. I'm using a MacBook Pro with 8 gigs of RAM, iTunes, Amarra, a Wavelength Proton DAC, and Ultrasone Edition 8 headphones. I don't have any trouble with 88.2/24 files - they sound just fine. Am I doing something wrong with the conversion? Some other issue, perhaps>