This is my desired use case as well.I don’t have an Ultra yet.
The ‘plan’ (possibility) is rca out to sub, optical out to balanced DAC, so I can use the balanced outs on the DAC to connect to the balanced ins on an amp, but obviously I’m wanting to maintain exactly the same functionality as if I used rca outs for L&R: volume control on the Ultra, bass management works the same, PEQ works the same, etc., etc.
A bit of MacGyvering, but at the end of the day, if it works as advertised, it should do the trick nicely. Not that the onboard DAC isn't a fine DAC, it is, but it's fun to experiment with various external DACs.