Oppo 105 and Hegel H160 DACs

Those are analogue, so you are using the Oppo's DAC. If you want to use the Hegel's DAC you have to use the Oppo's digital output into your Hegel's digital input. I would expect the sound to be very similar or identical. However, the hard bit is to do a proper comparison. Apart from the fact that it should really be double blind, you have to make sure that levels are exactly matched (within 0.2 dB). Also, the gain between the analogue output of the Oppo and the analogue input of the Hegel should be properly matched to avoid clipping. If you use the digital connection you may hope that you avoid this issue, provided the internal gain structure of the Hegel is correct.

To claim that the Oppo can be excellent pre amplifier is nonsense, It is very good blu ray that can be a decent CD transport but certainly not great one .Pure CD transport with quality coax cable will upgrade the the dound by far.

Any data?

Tired of people asking for data they know does not exist.

if you can’t  hear differences between DACs, then either your hearing is not as trained or skilled as others or your system just does not have the resolution to reveal these differences.

agree totally that oppo does not make a great CD transport.  It can be an okay CD transport, but others, such as CXC, will be much better. 

Plenty of reviews that disagree with you. And to be up to date, here is the recent Stereophile one for the new UDP 205: https://www.stereophile.com/content/music-round-87-oppo-udp-205 Measurement data would help, as no human can discern what an Audio Precision vel sim. audio analyzer can.