Upgrading streamer transport Lumin U2X Vs. Aurender N20

With USB, data clocking is handled BY THE DAC, so (IMHO) you will not get better SQ from streaming by spending $$$ on the digital source.

The best engineered DACs have isolation of the input, reducing noise etc. and buffer the data stream, which is then clocked into the actual DA processing by the DAC's clock, which, with USB, is independent of any timing errors of the input stream; hence, provided the streamer sends correct data to the DAC the sound is 100% dependent on the DAC (gasp), NOT the streamer. 

I believe, and it is my experience, that putting one's $$ into the DAC beats spending large sums on the streamer.  Others will probably disagree but with my Esoteric K-01XDSE, with a Rubidium external clock (feeding into a very highly resolving LA4, XA25, Atria II setup) there seems to be no dependence on the digital source component.  My preferred streaming service is Presto which is supported by the Eversolo T8.  Before Presto's streaming was turned on I was downloading from Presto into an Aurender's SSD.  I have lost no SQ moving to the T8.  Presto does not indulge in any processing of the digital data; other services may perform loudness normalization and even dynamic range compression.

Not sure if I will be using USB. Would like to keep my options open and will  try USB, Coax, SPDIF and AES/EBU. 
If I am using any connection besides USB, the streamers clocking, etc. will control.

If you end up with a Berkeley DAC, N20 is your friend. 
Otherwise try USB, AES and even the network card in Weiss. 

Everything @nubiann is saying is true regarding Lumin.  I was introduced to their products by an audiophile friend of mine.  I am currently running a D2, T2 and X1 in different systems.  Rock solid performance.  Sonically stunning.  Plays at the apex level.  Updates so fast and often even Roon can't keep up with Lumin at times.  Go to Lumin's web page and click on "support", under that click on "firmware release notes".  Every update they have ever done is listed and explained.  You are already a member of the Lumin family.  Sometimes we want to try something else.  Your call. Yes Peter Lee (I think that is his name) is not local but as stated they respond so fast.  Basically what I see Lumin doing is making the absolute best products they can without all of the "extras".  They are cutting edge.  They cut to the chase.  And they are extremely transparent in all that they do. Take care and happy listening!

If you are going to use Coax etc. then you need the best clock possible in the streamer and a premium cable.  I ended up putting 95% $$ into the DAC, 5% into the streamer!  Perhaps extreme but it works for me.

In general the optical plastic S/PDIF is is to be avoided unless you are in a noisy EMF environment.  AT&T glass seems to have disappeared, 25 years ago that was the ultimate connection.

Upper end Aurenders have first class clocks and they recommend Coax over USB; but that depends on the DAC. Often, only USB supports hi-res DSD streams, that is a limitation in some DACs.

Google the following:

what protocol is used by async USB connection to dac

it is an excellent technical description of the process, and explains why a well engineered USB DAC is insensitive to the data source.  Its summary:

"This asynchronous approach is why many audiophiles prefer USB DACs over optical/coaxial, as it significantly reduces timing errors (jitter) for better sound quality. "