Are all R2R dacs slow and soft?

Great that you’re cleaning up your streaming connections first.  The LTA could be a great choice, and that they offer a trial period is really nice along with that you can tailor the sound further with different tubes if desired.  I’d also have this Gustard R30 on the radar as it seems to offer a lotta performance for the price, and here’s a review where it wasn’t humbled compared to a much pricier Rockna DAC.  Best of luck. 

https://soundnews.net/reviews/sources/gustard-r30-dac-review-the-r2r-perfection/

+1 @zlone @audphile1 

You don't know until you try. And even if you change DAC's those possible improvements will still yield the same benefits with the different DAC. 

One may have doubts about filtering and switches when they read the blurb from PS Audio regarding their Airlens though: 

The PS Audio AirLens™ completely eliminates the problems of noise, jitter, and EMI influences through complete galvanic isolation and perfect reclocking of the digital audio signal. 

Noise from any number of sources including your computer, long runs of Ethernet cable, EMI interference through WiFi, the system modem and router, all contribute to increased jitter and a general reduction of fidelity in the system. Connecting your DAC via its coax or I2S inputs completely removes the haze and congestion caused by these noise sources.

I read this post yesterday and I, like many others, are still puzzled by what the OP is reporting. Those 10 lineal feet of ribbon tweeters can be quite revealing, thus why Magnepan provide resistors to tame the highs for some. But the PS Audio Direct stream converts signals to DSD, which has a reputation to sound more fluid and "analog" than PCM. From my quick readings I don't see anywhere people claiming excess brightness, and people in this thread who own this DAC seem to be even more confused. First thoughts; perhaps there is something "broken" with it? Perhaps the Audioquest Cinnamon HDMI cable is defective? Perhaps try a different connection interface? Well no, because the OP states it is only on a few tracks and otherwise its "very nice". And were these "soft and mushy" other DAC's demoed connected with the same Cinnamon cable? Hmmmm. Maybe they are poor recordings and the OP just has to live with that. I know I can't tolerate listening to most Jimi Hendrix music on my system for the same reason. Shame. 

So, ignoring PS Audio's claims of zero jitter or noise, try some filtering experiments suggested above, and in addition I would also try a coaxial cable or an all-copper "warmer" HDMI, as the Cinnamon is silver coated copper. Does your dealer have a Cardas coaxial to try? The warmest sounding digital cables I have read about are Pink Faun, not inexpensive though. 

I've only tried a handful of i2s cables. AudioQuest Carbon (normal hdmi, not dedicated i2s), Phasure HDMI^2, Tubulus Argentus, and now this Pink Faun digital interlink.

I'll be keeping the Pink Faun in the system and putting the Tubulus up for sale. Pink Faun has a lot of burn in to go (supposedly 300 hours).

Tubulus was really good. Between tubulus and pink faun, it's pretty close. Tubulus has better transparency and clarity up top. Pink has a bit more meat in the mids, more bass (although less tight), and more low end extension. Pink also has a slightly less forward presentation, and the highs have a bit more rounded edges; this is more pleasing to my ears with headphones like the susvara and TC.

https://www.head-fi.org/threads/high-end-cables-thread-digital-analog-power.937916/page-4

Pink Faun
 

Although I’m not a PS Audio fan, the DSD Mk1 DAC sounded very good and musical and was never harsh in my system. I doubt Ted Smith would have the new FPGA be less natural sounding and brighter than the old DAC. 
 

@traudio glad to hear you’re going to try the setup I recommended. Keep us posted please. 

@audphile1 

Why would we need a Ethernet switch if we're going to connect a long cable to it? Wouldn't be better just to go into the LAN Filter & not have an xtra item in the chain?

No.  The Esoteric XD DACs are R2R, using discrete precision resistors.

My K-01XDSE is certainly "fast", astonishing transients, and its resolution is outstanding - if the last note of a piece is a sustained piano note you can hear when the dampers touch the string(s).

There might be an issues with your HDMI cable.  I Googled asking about using your cable, designed for video, as an I2S cable.

"<>>Bandwidth Paradox: While high-bandwidth cables (like Cinnamon 48) are great for video, their faster signal edges can sometimes create timing issues (brighter/sharper sound) for I2S on certain DACs"

I2S is VERY sensitive to the cable as that cable carries data and clock signals from your Airlens to the DAC.  Your DAC is using the clock signal received over the HDMI cable as its master clock.

If you Google for "Cinnamon 48 cable causing problems on I2S DACs" you will specifically see treble brightness and sibilance as issues, and see suggestions. The consensus seems to be that ultra high bandwidth HDMI cables cause problems with IIS.

Try a cable designed specifically for I2S.