What an interesting question. You can definitely use the digital outs of the A8 to your DAC effectively limiting the use of the A8 to being a streamer. However, you need to specify ONE input and ONE output. So once you specify your digital source you can't specify an analog input at the same time. And once you specify a digital output you can't have an analog output at the same time. It is easy enough to switch your inputs and outputs in the app but your intended use isn't possible.
But also you would never want to do it your way since chaining sections like that will just increase distortion and degrade the quality of your music. Assuming your DAC has a way of controlling volume just take it's outputs and put them into your power amp.
Also, the DAC in the A8 is very good. I have compared it directly to the Gustard R26, PS Audio Directstream MKII, ANK 4.1x LE, and the Berkeley Audio Reference 2. The A8 outperforms the R26 and Directstream, may or many not outperform the ANK 4.1 depending on what is important to you, and is just very slightly worse than the Berkeley Ref2. In general the A8 is an extraordinary value although there are still some rough spots in the software.