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[Lowland]

Yes, you can make them sound a bit nicer with more watts but that can become very expensive for small improvements. I think a D200 with a pair of Contour 1.1s would sound more than nice enough to satisfy pretty well anyone who doesn't obsess about getting the absolute ultimate in performance because that is going to become very expensive very fast. Provided the room isn't too big, 200w/ch with the stand mount Contours is capable of delivering superb performance with good setup. If you have a D400 they will sound nicer but you're going close to doubling the cost of your amplification and you won't get anywhere near twice as good sound quality and you're also probably never going to use the extra 3 dB of peak output level you can get from the extra watts. You're definitely into diminishing returns territory in my view.

You're pretty much spot on @David A. 

I had (still have in a box) a D170 on my C1's first. Then went to D200 and SAM with the firmware upgrade and that gave the biggest improvement. The step to Od'A did give me better sound quality indeed but the jump wasn't as big. It's mainly at lower volumes that the sound becomes fuller with the Od'A but I'm sure larger speakers at the end of a D200 would have done the same if not better. I just jumped for the Od'A with the prospect of bigger speakers in the future. My previous post was more to illustrate how much influence good amplification (Devialet   )  can have on a pair of speaker.

Thank you both for your informative replies.
Upgrading to D400 wasn’t on my mind before. Now it tickles my fantasy. 
It would come down to buying a second hand D200. 
The pro is that prizes are dramatically reduced. The con is that there’s less opportunity to find out how it will sound like.