Upgrade DAC Power Supply, Streamer, or ?

I would upgrade the streamer to one that supports asynchronous USB connection with your DAC.  Avoid using Coax and Toslink.  IIS is a popular connection BUT it requires a very high quality source and IIS cable.

I researched your DAC and it has galvanically isolated, buffered support for async USB.  This is optimal except at the very highest level of IIS (I2S) implementations.

IMHO that will yield far greater SQ benefits than any front end tweaks.  The DAC designers appear to have applied much thought to creating great sound, but your coax connection from an entry level streamer is probably holding the quality back from where it could be.  With Coax (and IIS) your SQ is strongly dependent on the source as the DAC is affected by jitter on its input.  With a USB connection the data is buffered and isolated and the DAC's clock is responsible and so you are largely independent of source issues.

For $2K you have many options, I currently use the Eversolo T8 - about $1,350, (feeding a $25k DAC).

@foamcutter, I’ve got aftermarket SBooster LPS units on both my DAC and my streamer/server, and in my experience they’ve been a sound investment for my system. 
I would say that before you run out and spend $2,000 for a power supply solution for either your DAC, streamer, or both, I’d make sure that both units are the ones you intend to live with for awhile, like first having the very best DAC that you can afford and then going from there. 
 
If you plan on sticking with the Merason Frerot DAC, I think that getting rid of the stock walwart in favor of at least an iFi SMPS(switch mode power supply), which aren’t too expensive, or something similar, would make a noticeable difference in sound quality. 

As @lanx0003 mentioned you’d run the USB out to the USB in on your DAC so the Zen would be fine assuming you’re gonna use the DAC. 

As @logistics mentioned, a power conditioner is a worthwhile addition. My listening experience after adding an Audioquest Niagara 1200 is:

My immediate reaction was that the quality of the sound seemed calmer, but without any loss of pace or tonal richness. The overall character of the sound remains the same. Upbeat music still remains upbeat, but calmer music feels even more calm.

There is slightly less edginess to the quality of the sound but to a perceptible degree. My system was already pretty good in terms of not sounding harsh or edgy, but the AQ Niagara tones it down a bit more. With the AQ Niagara 1200 in place, listening fatigue seems to have gone away altogether for me.

I'm better able to follow individual instruments and voices when they are layered together in music. In complex music passages in some tracks, I'm used to voices and instruments sounding mixed together. The AQ Niagara 1200 seems to provide better separation and more defined placement of individual voices and instruments.  

More "air" around instruments & voices. Better able to hear the acoustics of the recording space: On the Cowboy Junkies, The Trinity Session

• Highs seem sweeter: 

• Better flow.

• Tonal quality sounds more evident: 

• No loss of pace or timing.

• Bass seems fuller.

Put all your files on a NAS, get rid of the computers altogether, and buy the best streamer you can afford. At $1800,The Eversolo T8 is the best thing you can get under the $5K HiFi Rose RS130. But you have to use the SFP to realize it’s best performance. Which means buying a decent switch with SFP out.