It's actually a toss-up. Upgrading your speakers will make the biggest difference in the sound of your system but you will not hear what they are capable of until you upgrade your source components. Conversely, upgrading your sources will make significant changes as well but to get the full benefits you will have to upgrade the speakers. This is why I always suggest that once your system is matched, leave it alone and enjoy the music. Otherwise be prepared for a trip down the rabbit hole.
Upgrade speakers or DAC
Next upgrade is going to be either Front L/R speakers or streamer. Current speakers are Sonus Faber Olympica III, looking to upgrade to Il Cremonese. Streamer is Cambridge Audio Edge NQ looking to upgrade to dCS Rossini APEX.
Which upgrade will make the most significant improvement?