"Did you expect the C15 to sound as good as 808.3 which is double the price?"
Well, why not? The 808.3 is in fact a CD player,
I would say that the 808.3 is a preamp that happens to have an excellent CD player built in. The 808i.2 is an excellent CD player that happens to have a preamp built in. The 808.3 does so much more than spin discs. The analog input is unbelieveable and the ID40 Sooloos interface is the creme de la creme. The C15 is a different animal that is notable improved over the first two iterations of the C10, but is not the same device type as the 808.3. So, no. I would not expect a C15 to sound as good as an 808.3, irrespective of the price points.
The original Sooloos model was to have each function in a separate device (source, storage and control). M has combined them in the C15 while also preserving the option to have separate devices with the other new Sooloos devices. Some might call that innovation.