D/A converter built into amp.

Separate components versus 'integrated' solutions can be wonderful and there is always a compromise.

The big compromise is capability and features versus cost.

Could a stand alone DAC fix it - possibly.  What is the cost you are willing to spend?  And then do you want to also explore a separate streaming and DAC option?

Why aren't you using USB or Ethernet inputs??

I have the Bel Canto one streamer which does not have USB input except for files.   It is Ethernet connected. The optical cable goes from the streamer to the amp. The streamer cost $1700 so I would consider anything under $3000 as appropriate.

At least try the SPDIF Coax connection before moving on to outboard DACs, which are a bottomless pit of options.