We are fortunate to probably have sold more Sapphires than any other dealer in the world. That being said, with our experience we have found numerous different synergistic combinations. Certainly, like all Dynaudio loudspeakers, you need clean high-current amplification to make the most out of them but you really have options to suit your tastes. Some of our favorite pairings include Soulution, Naim, Octave and Burmester. It is really fun to demonstrate the Sapphires on one of our reference rigs (Soulution or Burmester) and high value rigs (my favorite Naim Supernait, HiCap, HDX, DAC, 555 PS) and both systems, completely different price-points, are simply inspiring!
The C4s and Sapphire are more different than similar. If you primarily listen to Classical music and have a large room the C4s are the right ticket but for nearly every other application I personally prefer the Sapphire. Be warned though, the Sapphire can be utterly revealing so finding one of those synergistic pairings is key to ensuring your enjoyment.
Although both are amazing speakers which most could live with for a lifetime.
Cheers!
Tyler