Gustard R26 vs hi end streamer

As I mentioned earlier, the bass improves, which is the first noticeable change within the first week. The piano notes gradually settle as well, but the timbre and overtones of the violin sound unnatural and somewhat raspy. Sibilance isn’t well-controlled either and I'm not sure if it could be further tamed (feedback please). I have the time and patience, but I don’t want to wait 200–300 hours only to find that the tone is still unnatural, as some have reported. That’s quite a lot of burn-in time for this R2R DAC.

And you're certain that the sounds that you hear are coming from the R-26 and neither the Cambridge integrated or Cambridge streamer?  I've not read any reviews that indicate the issues that you've had.  

DACs use electrolytic capacitors in power supply that need a very long burn in. Amplifiers need less time. In my experience with Chord and SMSL DACs it was 3 weeks plugged in AC. And height frequencies finally improved at the end of this period.  

Both Cambridge int amp and streamer are endorsed by Stereophile so there is no need for concern.  I stand by my previous statement, i.e., at this point, R26 sounds good but performs at most as well as MXN10 internal DAC.  Both are neutral but I expect the R26, with its r2r topology, to sweeten / smooth the midrange / treble better than the ESS-based delta sigma DAC in MXN10 but it doesn't.

Give Maria Duenas's II. Larghetto Beethoven Violin Concerto in D major, Op. 61, in her 'Beethoven and Beyond' album a listen (24/96 in Qobus) and let me know how you feel about the violin timbre. 

The R26 sounds best when using I2s or USB inputs, in my opinion.  The Cambridge only has coaxial and optical outputs if I'm not mistaken. 

I've streamed with the renderer in the R26 using Audirvana software. I certainly wouldn't call it awful.