I have the C-658 and have been using it for a year or so. I currently use it with a FirstWatt F6 as a streamer, DAC, phono stage (MM cart) and believe it to be the best bang for the buck out there. I also have the HDMI card installed and run a Panasonic UB9000 through that. It all sounds great to me. I previously used the C-658 with Rogue M-180 tube amps, a couple vintage power amps (Onkyo M-504, Sony TA-N80ES), all going into Focal Kanta No.2s. I have compared the DAC to an Auralic Altair G1 using a Yamaha CD-S1000 as a transport and I believe there is a slim line between the 2 devices as the G1 does add a little to the overall listening experience but again, at a significant cost increase. Are there better DAC/preamps for function and sound? Yes. But, IMHO not for the same monies.
I was considering selling mine but since I hooked it up to the F6 I may keep it as part of a 2nd system. It certainly is versatile.