The Marantz should make a fine transport. If you like its operational features, keep it.
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Having previously owned a DAC 1 (that I used with a Marantz CD player as transport), and currently owning a CX-7e, I can say that the presentation of the 2 solutions is not at all similar. Both should be very good, but the Ayre is more focused on extension, soundstaging and "air" around the instruments, whereas the Bel Canto is more on the romantic side of things. Both are very musical.
Personally, I obviously prefer the Ayre's presentation and have a lot of fun every time I listen to a CD on it, but the Bel Canto would be a good choice if you're looking for a more romantic sound or would have use for its imputs and its ability to operate as a preamp.
So I guess that if ability to operate as a preamp and ability to play SACD and DVD-A are immaterial to you, I would expect better redbook playback from the Ayre.
Keep in mind, though, that I haven't heard a DAC 3, only a DAC 1.