C47 has an issue with the USB input to the DAC - it sometimes cuts off the first 1-second of the beginnings of tracks. Official word is that won't be fixed but is fixed on newer versions. I use the Coax digital input and that works fine (no cutoff) with PCM up to 192/24. The optical input also does not have the cutoff problem and I've run that at 192/24 rates as well. If I need higher rates or DSD (which has not be the case for me so far) I need to use the USB input and put up with the issue.
Also, the C47 does not have a tape loop / monitor feature.
So if use of USB and higher rates (above 192/24) is important to you, you might want to look for something newer (with a DAC2 module) or get a outboard DAC. Otherwise, C47 is a great sounding preamp. I've not heard a MA6100 or a P04, so cannot comment on sound differences but would guess the C47 would sound much better.