Denafrips Ares II Buffering and Reclocking

I have a Musician Pegasus that is pretty close in design to the Denafrips Pontus ll so my experience may be relevant to your situation. I recently added a Denafrips Iris DDC mainly because I wanted the ability to take advantage of my DAC’s i2S input. Using the USB input on the Iris and i2S the improvement was immediately apparent and NOT small. Given the dramatic improvement I heard I wouldn’t be surprised if you may also hear improvements going from USB to SPDIF and definitely think it’s worth a try. You could buy a used Iris (which is what I did) and if it doesn’t work out sell it for little/no loss or buy a Musician Pisces (similar to Iris) through Amazon and return it if you don’t get meaningful improvements.
https://www.amazon.com/Pisces-Interface-STM32F446-PCM768KHz-Isolation/dp/B0B7HPTMFW/ref=sr_1_1?crid=I5M5SWOUZCF&keywords=musician+pisces+ddc&qid=1668444125&sprefix=Musician+Pisces+%2Caps%2C80&sr=8-1&ufe=app_do%3Aamzn1.fos.2b70bf2b-6730-4ccf-ab97-eb60747b8daf

Note the Iris/Pisces do not have an SPDIF input so if you need/want that you’d need to step up to the Hermes or the new Musician Leo that again is likely very similar to the Hermes but considerably less expensive. My experience aside, despite whatever buffering, etc. exists in the Ares’ input I don’t think Denafrips would’ve gone to the trouble of developing multiple DDC converters if there weren’t more significant benefits to be had. Anyway, just my $0.02 FWIW, and best of luck.

If all the digital inputs are feeding into the same FIFO buffer, then there's no reason why one connection method should sound different from another.

Per Stereophile's measurements, you might have some issues with full-scale signals overloading the oversampling filter for PCM audio.

Also, depending on how Denafrips implemented DSD decoding, one bitrate may produce more distortion than others, but it's likely not going to be audible. See this blog post for more information.