DAC Choices - $5,000 to $7,000

In my world, I2S out of the Ambre has been the best sounding input into the Adagio, followed by AES/EBU, with S/PDIF and USB being less desirable. To my ears, using Ethernet from the Roon Core into the Endpoint (i.e., the Metrum Ambre), and having no USB interfaces in the signal chain has been a sonic improvement - and yes, I did try some pretty good cables (i.e., Totaldac and Curious) as well as all the usual suspects in the world of small box USB do-dads.

Since I also own a Jade, I did pre register for the DAC3 upgrade but wasn’t clear whether that upgrade also affected the Adagio/Pavane, although I let them know I own both.

I have no experience with the Gustard U12 but I see it is a USB (input) converter so I assume your source is a computer. If you are able to use a direct Ethernet connection, the Ambre works great and was sonically comparable when I owned both it and the SOtM Ultra trifecta (slight differences but both units sounded good). The I2S output connector from the Gustard uses HDMI so you will need to adapt that to RJ45. Hopefully the Gustard or Metrum folks can help you with the wiring. Here is a review of the Gustard U12 with measurements, in case you have not seen it.  BTW, if you heard positive improvements by using the Gustard U12, the Denafrips GAIA should perform very well for you since it looks like a serious piece of gear and quite versitile.

Amber seems an obvious choice but I do not want Roon. One of my friends is using modded Amber with great success.

I have built a computer audio transport in a Streacom FC10 chassis, powered by HDPlex200 LPS and OS is APL4. I have installed Cantata and use window10 to remotely control my transport. This works very well for me. But the problem is the USB input of Adagio which I want to avoid.

U12 is only temporary till better quality GAIA or something similar is added. U16 has good reviews but GAIA has better specs on paper. SOtM is out due to the hassle of multiple boxes and their power supplies. Audio-gd DI-20HE is also out because of its i2s connector being RJ45. 

If you can live with the boxes, you can sometimes find all three SOtM Ultra units (including the switching PS, which is pretty good) being sold at a good price.   They did sound good but in a full-bodied sort of way with maybe a smidgen less resolution than from the Ambre.  Both were grainless and musical.  It was really a pretty close comparison with the Ambre, and I owned both units at the same time.  The Ambre was just a little more dynamic and hard-hitting and the SOtM a touch richer sounding. However, they were so close I chose to keep the Ambre because it is only one box and because it dropped Roon less often (from seldom for the SOtM unit to pretty much never for the Ambre) and was easier to restart after powering down.  The GAIA looks like a nice unit.

BTW, how did your friend modify the Ambre and in what ways did the modification make the Ambre sound better, if you can share? 

SOtM boxes are out because I already have many "boxes" from a cassette player, TT to monoblocks. I had to modify my rack a couple of times to accommodate these boxes.
Most likely I'll go for GAIA to be future proof and to avoid any modification of my custom made server. GAIA has many input/output connectors and even both i2s types.

My friend has done modifications to both hardware and software of Ambre. He used sd card to flash Ambre.

Loving my Holo Audio May DAC - makes digital sound analog (in the best way possible).