I upgraded from a MA6500 to a combo of C200 and 402. While the 6900 is above the 6500, it is not by much so, so I think my experience may be relevant for you. Yes, the difference was dramatic: both from the preamp part of the 6500 to the C200, then from the power amp part of the 6500 to the 402. You get much better soundstaging and better defined instruments, even more natural voices, and a much better defined, stronger bass.
Regards,
Florian Hassel
P.S.: I have been bitten by the upgrade bug,too, and contemplate about when best to upgrade the 402 to a pair of 501s...
Regards,
Florian Hassel
P.S.: I have been bitten by the upgrade bug,too, and contemplate about when best to upgrade the 402 to a pair of 501s...