Synergy strikes again….
I have been playing around with a MATRIX xpdif-2 and Singer su-6 ddc in my main 2 channel system. Both units offer positive improvements that can be heard/understood if you focus your ears/attention properly on known tracks and really listen. Unfortunately in MY system both negate the specific improvements/sonic character my FTA USB cable brings into the system and this has been a back and forth experiment for days and many many hours allowing myself to enjoy each set-up for its own distinct flavor trying to figure out my preferences.
Here is a break-down of sonic differences i have been able to note on this current system consistently.
Innuos sense 2.0 > Innuos ZENith Mk3 > FTA Callisto direct to Lampi Baltic 3 DAC
Thus far this is my preferred combo with current amplifiers and tube combo. Grand soundstage, well behind the speakers and extending out on both sides (on most tracks it is difficult to isolate the speakers). Excellent depth and sharp imaging with really good instrument separation. This particular USB brings out a depth and presence in the bottom end that I find VERY pleasing and helps warm up the room which happens to be a bit oversized due to the really tall ceiling. It’s not easy to describe but easy to “feel/understand” by simply swapping out the USB and then you just kinda “notice it” 
Innuos sense 2.0 > Innuos ZENith Mk3 > Singer SU-6 using AES output to Lampi Baltic 3 DAC
Thus far every DDC i plug in seems to have a blacker background, which allows for more detail and a small improvement in separation, this is across the board and is pleasing. With the Singer SU-6 soundstage width is excellent, almost as wide as my preferred combo and it takes an unusual amount of playing particular tracks to notice the very small difference. The overall performance of the system though leans out with the current configuration, There is nothing bad or weird about the sound, it’s simply leaner and less visceral in the room. Excellent performance no complaints, relatively inexpensive performance upgrade. I have not had a chance to try different cable or output combinations yet. I did NOT use the stock power supply, I immediately used an iFi unit, I won’t bother w/ stock power supplies at this point if I can swap out, I have a small stash of iFi units (elite, X and power2) and like them.
Innuos sense 2.0 > Innuos ZENith Mk3 > Matrix X-SPDIF2 using AES output to Lampi Baltic 3 DAC
I think this unit is a champion at its price point, I have 2 of them and in direct comparison to the Singer SU-6 the only differences I have been able to discern between the 2 units is that I feel the Matrix may be a bit warmer overall, just enough so that if normally your components ride on the side of overly bright this would possibly take the “sheen or glare” off while the Singer unit might enhance those characteristics.
Neither unit as tested allows the visceral feel of the music to shine through like what the FTA Callisto USB going direct from the streamer to the DAC does and this is where my quandary lies, I take a hit on overall detail presentation by not using a DDC but the system simply sounds “better” to my own preferences…
Lastly, I did also try out the SoTM usb regenerator, it’s a very nice unit but it brought the soundstage in way tighter than is my preference, with the SoTm unit in my main 2 channel using the Lampizator DAC’s i lost too much soundstage width past the speakers to keep the unit in place. (it felt weird) Surprisingly though that is NOT the same effect it has with my other DACS in the smaller room, quite possibly because of the size of the room the soundstage does not feel as restricted while i also receive all the benefits of a blacker background, thus the SoTm and the others reside elsewhere in the house fully integrated and performing their duties.
In conclusion, I find each and every DDC I own worthwhile, they all help bring out detail that is otherwise lost, I use one in each system except my main 2 channel rig. I have an almost OCD level of preference for making my speakers disappear and having a particular sense of space in the music. With both my Amber 3 and Baltic 3 from Lampizator that preference was affected just enough that I mostly do not care for the benefits provided by any of the DDC’s and would rather listen without them in place. I have worked hard to tune my main 2 channel rig just right for me and not every upgrade is necessary or worthwhile at this point. Happy listening all, YMMV