It is important to have clarity when posters discuss a "streamer" or a "server" in these threads. I believe there is sometimes confusion over a poster’s intent without clarifying whether they are talking about a server, a streamer, or a combined server/streamer unit.
If Roon is involved there can be a separate core/server and a separate "audio device" (sometimes called a streamer, endpoint, renderer, or network music player) playing Roon Ready. These two functions can also be located within the same device as with the Antipodes K50 or Oladra, Grimm Audio MU1, or Roon’s own Nucleus. Other devices like the Grimm MU2 might add a DAC and sometimes a preamp for a complete one-box digital source solution.
There are also pure streamers (some with file storage) that can run Tidal and Qobuz through their manufacturer’s own streaming platform, without using Roon. Examples would include Innuos Pulsar and Auralic ARIES G2.2, just to name two.
At the lower price end, Roon’s own Nucleus One offers a one-box solution that outputs directly to a DAC via USB or, you can connect it to your network and control an outboard streamer, which should arguably improve sound quality. The Nucleus One is basically a computer, as are all of these streamers/servers, it has an internal fan, and it costs only $500.
Amazing to me is the contrast between the detailed discussions on these threads (and folks spending $10K+ on server/streamers) vs. how esteemed reviewers such as John Atkinson and others at Stereophile are conducting reviews of expensive DACs connected directly to the USB output of their Roon Nucleus+, which is basically a computer with an i7 processor. Reviewer Srajan Ebaen at 6moons uses his iMac as a server in his main system, but at least he routes the output into the Singxer SU-6 prior to the DAC.
I have had multiple music servers, and a couple that were one-box server/streamers (from Antipodes and Mojo Audio) and I have yet to hear a difference between any of them. When I did start to hear differences was when I separated the "server" from the "streamer" by connecting the server to my network in a separate room away from my system, and then connecting the network to a streamer (currently via optical fiber) in the same room as my music system. My first ah-ha moment was when I started using Sonore’s Signature Rendu SE Optical flagship streamer. I was surprised at the level of difference/improvement that provided over the Metrum Acoustics Ambre streamer that I was previously using. The Ambre is based on a Raspberry Pi board and should be no slouch because of its onboard linear power supply and femto clocking, but the Sonore was clearly a step up to my ears.
I am currently using SGC’s sonicTransporter i9 (Gen 4), which is a computer, as a networked server and cannot hear any difference between that and the Antipodes or Mojo Audio servers that preceded it. The sonic improvement comes from routing that signal through an outboard networked streamer (Sonore) prior to my DAC. The next even bigger difference I hear is between different DACs, but that is a different topic.
Regarding DDCs, I have had the Gaia here previously, and I currently use a Singxer SU-6. I power the SU-6’s 7.5 million uf supercapacitor using a linear power supply and not the wall wart it came with. I began using the DDC solely for the flexibility of connecting multiple DACs via something other than the USB output out of the Sonore Sig Rendu SE O. However, the Singxer also provides galvanic isolation as well as reducing jitter to a low level. It certainly cannot hurt to send a lower jitter level signal into your DAC. In addition, optimized USB inputs can be pricy and not all DACs do a great job with their USB inputs. By having the ability to try multiple input connections to your DAC, you can select the input method that best optimizes the sound of the DAC.
IME, anybody expecting a significant sonic breakthrough by adding a DDC will probably be disappointed. But adding a good quality DDC certainly shouldn’t hurt the sound quality of a digital front end and might make a marginal improvement by isolation, lowering jitter, and optimizing the connection to the DAC.