This is a review and detailed measurements of the WiiM Mini Wifi/Bluetooth streamer. It is on kind loan from a member and is on sale on Amazon for US $89. As you can see the former factor is a "hockey puck." An included phone charger needs to be used to power the unit over USB-C. It did not...
www.audiosciencereview.com
The WiiM's DAC isn't impressive by today's technical standards, but those standards far exceeded most of our hearing a long time ago. It's fine by any reasonable standard. And what is the S/N of your tube preamp? Worse than the WiiM probably.
Spend $84 and try it out! If you don't find the DAC good enough you can use the bit-perfect optical output into the DAC of your choice later.
FWIW I am using 3 WiiM minis, two with optical out and one with analog out. Personally I can't tell the difference