I wanted to chime in as I studied this and other threads before upgrading my digital front end recently. My former streamer was the BlueSound Node130 with upgraded power supply, quality cabling, and was hardwired to ethernet. The BlueSound was going into a Denafrips Terminator Plus via coax.
After a ton of research, I found a member selling a Lumin U1 and decided to take the plunge. The rest of setup hasn't changed besides the Lumin, first feeding the DAC via coax and after appreciating the improvement. switched to AES. The difference was not subtle in terms of improvement versus the Node.
Biggest change by far is the improvement in low and mid-range response, the bass is much more full and the mid-range filled in much more so with the Lumin. Other improvements are what fellow members noted like better detail, blacker background, better separation of instruments and layering, and the soundstage was improved.
So even if you have a good DAC and other gear, a streamer definitely brings big improvement at least for me.