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I wouldn't. In my lengthy and boring experiments with disc spinners and DACs, I came to the conclusion that a DAC in a half-decent CD player is more or less the same as that in a DACMagic, and it's the transport mechanism (ie computer versus spinning disc) which has more of an effect on the quality of output. If you had an amp with balanced inputs that might be a different story, but you don't - I suspect you'd get a lot more benefit from spending 250 quid on upgrading your amp than getting a DAC.
If you wanted to use the DACMagic for improving the sound quality of other sources like cable or streaming then that's another matter, but tbh I'd start by plugging those into the DAC in the Yamaha (perfectly good quality). Do you have the A70 and the 640SE hooked together via preouts?