@yyzsantabarbara - I remember hearing about Playback Designs and their buffering technology at least 10 years ago. Amazing seems to be a frequently used descriptor. Many praises about the SQ. Pricy though for the new stuff - Playback Designs MPS-X Streamer, $11,500. American company, certainly worth a look.

BTY, I would be comparing any other streamers that I try to a Sonore Signature Rendu SE optical that I picked up a few months ago.  As you said, smooth - like butter on a hot biscuit.  I am currently running the USB into a Singxer SU-6 DDC for purposes of connectivity flexibility.  The SU-6 doesn't change the general character of the Sig Rendu and it certainly doesn't hurt SQ.

Might I put the $5195 Rose RS130 Network Transport up for consideration.

Just put in an order for one after my local dealer let me take their demo unit home for a week. Although Rose has their own app, I used it as a Roon endpoint. Trialed it by both Kimber D60 Coax and Curious USB into my Rockna Wavelight DAC. Also tried I2S, but only had basic non audiophile grade HDMI cable on hand with which to give it a go. Via USB was nice. Via Coax was truly impressive. By comparison made my very competent Bricasti M5 seem congested. Once some things are heard they cannot be unheard. Much more air. Expansive soundstage and imaging. Simply no listening fatigue. And on top of that the unit is drop dead gorgeous. My system lives in the living room, and I had my wife’s full backing when I placed the order. If my dealer would have allowed me to purchase their demo unit I would have held on to it. Most likely will be April by the time I get my unit. The Rose has an OXCO clock, and my thinking is the benefit of that clock is more apparent with synchronous outputs all other things being equal, explaining why my experience with coax was so much better than with USB. My guess is that with a decent I2S cable in my case it would be better still. The Rose allows for connection to fiber via SFP, something I’d like to do down the road. You can also use an external clock if so desired. I very much liked the potential upgrade path.

SoTM is good for Room.

Good thoughts on the Rose.  Their units have been getting positive feedback lately.  I have owned the Bricasti M5 and while I like the Signature Rendu better, it is still a good unit so bettering the SQ of the Bricasti through the Rose is an achievement. 

I have found that the Melco Server/Streamers in Roon Mode offer some of the finest streaming sound quality. You would be looking at the new Melco N5 - the second from the top Melco N1 model. These are not cheap but nor are they super expensive streamer/servers compared to much of the competition. See if you can get an N5 (or an N1) home to try in your system. Contact Luxman America for your closest US Melco dealer.