I have 3x M200 that all experienced the same "dirty" audio, mostly when they were just powered up, but it steadily got worse. I found that C313 was bad on all 3 units. Once replaced, they are back to normal. I suspect this issue may come back in another 6-8 years, but now I know how to fix it. C313 is a 100uf 35v electrolytic capacitor. All 3 measured about 24uf when tested.