Everyone is entitled to enjoy what sounds best to their own ears. I have built systems using the Pi B+ and Pi 2.
USB DACs have included:
nuforce.com/index.php?optio … /index.php
and a few generic PCM2704 boards in the $8 range
I have also built a box with the i2s DAC:
audiophonics.fr/audiophonics … -8396.html
My source material has been “ripped” CDs streamed from a local NAS, or web radio, or Spotify.
I have used digital amps (TPA3116), a tube amp (12ax7, 6BQ5), and some older solid-state amps. For speakers I have used a variety of home-brew systems and my old Klipsch Heresys.
My opinion would be that your source, amp, speakers and listening space will have a larger impact on sound quality than any of the DACs above. Add an equalizer to fine-tune any slight differences if you want.
One nice thing about the USB DACs is that they can be treated as connectors between your Pi and your amp, and can be easily “rolled” just as I might roll 12ax7 tubes on my tube amp. Most USB DACs also allow Volumio to control volume in the hardware mode, which might be convenient for you.
On the other hand, my next box may be a portable, using a Pi A+ and the above i2s DAC, because that combo would be cheap and compact.
In my opinion, all sound quite good. Pick one (or several) and have fun!