Upgrading streamer transport Lumin U2X Vs. Aurender N20

Aurender support is solid. I have first hand experience after a firmware update bugs. They addressed the issue I reported immediately. 
Aurender N20 features the best clock available - OCXO that ensures the highest quality output signal

 

OCXO Clock Controlled Outputs

Aurender’s Oven Controlled Crystal Oscillator (OCXO) clocks are unique in a category that has historically treated superior SPDIF and AES/EBU audio outputs as an afterthought. Unlike USB, where the DAC’s clock matters most, SPDIF and AES/EBU data streams are only as good as the transport clocks that govern them. N30SA, N20, and W20SE all employ exceptionally accurate and stable OCXO clocks for a smooth, saturated, and laser-focused audio presentation.

I do not find a y specifics on clock used in U2X. 
 

Also, having owned both Lumin and Aurender, the Aurender Conductor app is worlds better than Lumin’s native app which I found extremely user unfriendly. 
The native app becomes immaterial if you use Roon

This should be a meaningful upgrade for your system.  I'm not familiar w/ Lumin but I own an Aurender.  Aurender is designed with the AES/EBU output optimized. I hate to throw out something that you didn't ask about but Inuous is very comparable with Aurender and is optimized for USB output.  If your set on using a USB input in your DAC I would take a good look in the Inuous line.  They do a great job with USB.

Just for the sake of accuracy,  Aurender N20s don't come with storage loaded onboard.  Rather, they have slots where the owner can add their preferred SSD.  Good luck with your upgrade.    

An N20 makes perfect sense. Keep your DAC for a while and buy a 10mHz master clock for the N20. This will take your DAC to a new level and give you time to re-evaluate your system, don't cheap out on cables and give the N20 about 4 months to break in. USB sounds just  as good as AES don't listen to the naysayers.