I used to sell the MMF-25 and I currently sell the 640C so there is my disclaimer. I think the old Cambridge 300 kicks the snot out of the MMF-25, the old Cambridge 500SE kick the snot out of the 300, the new 540C tops the 500SE, and the new 640C tops them all by quite a bit. In fact, the 640C is up with the Arcam CD-73t in terms of performance, which is not surprising since they use the same Wolfson DAC (the same DAC used in the CD-33t that Stereophile just said it doesn't get much better than, at any price - granted, the CD-33 uses four of them instead of one, but hey).
Also, the Cambridge uses an incredible Sony transport mechanism.