[quote user="vinod_david"]is it mandatory that 640 should be coupled with 640 only? what if i couple 640C with 540A, will that affect the overall perfomance ?[/quote]
No, not at all. Yes, the 640C is a better player than the 540C, just as the 640A is a better amp than the 540A, but as Clare said it makes sense to spend more on the source component for the best performance, as information lost at that point can't be recovered in the amplifier.
So the 640C/540A combination will work very well indeed.