You could connect a third party DAC to the Line-In, but it won’t ‘improve the quality’ unless the DAC has some euphonious (read: inaccurate) sound characteristic of its own which you appreciate.

Conversions can only degrade, they can’t resurrect what’s been lost or create what never existed in the first place.

Thanks for the reply!

I am streaming mostly on Tidal Master. I know that Sonos does not support MQA decoding natively. I see that some external DACs support MQA decoding, would it help if I add a DAC with such function to the Line-in before the Sonos Amp?

I very much doubt it. Firstly the ADC on the Amp’s Line-In will digitise at 44.1kHz, lopping off any ‘unfolded’ (and inaudible) ultrasonics. Secondly MQA is regarded by many as a fundamentally bad approach.