I've owned the Onyx for about 3-4 years now. I discussed with Cees the designer of both the Onyx and Morpheus and he mentioned by upgrading the Onyx DAC chips to the level 3 chips the sound of the Onyx would be very similar/close to the Morpheus so I upgraded the chips.
The sound with the DAC 3 chips is more dynamic with a bit tighter bass control. The Onyx with the DAC 2 chips is a little more laid back and slightly warmer in my opinion but the differences are minor so it may depend on what type of sound you are looking for. I'm using the I2S connection to a Metrum ambre streamer.
I kept the DAC2 chips and did not trade them in case I ever want to switch back and forth.