I noticed one of the album's CD rip has screeching distortions and pops under ZX507 - never noticed that before.
I replaced it with official 320K AACs, and it sounded great. I still need to break this psychological barrier of lossless vs lossy thing: there is nothing that special about CD quality and quality achievable with 320K AAC/MP3s. The ability to losslessly recover the original audio is albeit a nice feature, but if the transparency argument is to be trusted, fear not to add AACs/MP3s to your collection.