With a RB Clip+, UHA, low gain, volume knob at 1:00 the HD600 sing. The UHA is definitely more powerful with greater punch and over all better sounding than the C421.
If I set the gain to high, I can only turn the volume knob a few degrees up as it gets way too loud.
For those that have this amp/dac, is it better to get a coaxal cable and put one end in the HO of the Clip and the other end in the UHA COAX? Now a 3.5mm cable goes from the Clip HO to the UHA input.
No because Coaxil is intended to use the UHA6 MKIIs DAC and bypassing the sources DAC and Amp section. So unless you get a transport or a DAP with Coaxil/Optical you can't use it.