KaiS contributed already to the topic, here for example in terms of D/A filter of ESS/AKM:
https://forum.rme-audio.de/viewtopic.ph … 73#p218673
https://forum.rme-audio.de/viewtopic.ph … 98#p212998

There are only subtle differences in terms of D/A filter, but no differences in terms of "quality differences".

Maybe more interesting to look at the features of the different devices:
- ADI-2 DAC FS has definitively a different feature set compared to ADI-2 Pro FS R BE
- ADI-2/4 Pro has additional features on top of the ADI-2 Pro FS R BE

Manuals and product pages on RME's web server provide further information.

Additional help by this sticky posting:
   https://forum.rme-audio.de/viewtopic.ph … 06#p165706
and my blog article:
   https://www.tonstudio-forum.de/blog/Ent … ses-EN-DE/

Short overview, as I think that this isolated question alone about sound might not be sufficient for a final choice.

The ADI-2 Pro was the first device, intended for studio use but also been used by some HiFi enthusiasts who like the feature set. Some examples: extreme power headphone outputs to drive even headphones with high impedance, auto reflevel, dynamic loudness, PEQ to normalize headphones, etc.

The ADI-2 DAC followed to optimize the product for HiFi
- using reference (output) levels which match better for HiFi devices
- removing digital I/O and digital outputs which are not necessarily needed for HiFi
- removing analog inputs
- instead of two phones outputs and support for balanced operation, one phones output or iem.
An additional intention was, that HiFi user should not pay for "studio" stuff, that they most likely won't require.
So many years ago the 1st ADI-2 DAC FS could be released with a price below the "magic" €1000 vs €1650 for the Pro model.

Later models of the ADI-2 Pro also got FS clock and a remote, because it turned out to be useful and because some users wanted to have the Pro version, to have also analog ins, balanced phones support, etc..

The ADI-2/4 Pro FS got additional features, see the sticky posting. One reflevel more (5) combined with auto reflevel optimizes dynamic and SNR over an even wider range. The rear TS outputs changed to TRS for balanced operation plus the feature to have an independend playout path for e.g. a mastering chain, demanded by mastering engineers. Besides this some more very useful features went in like the possibility to connect a turntable and an excellent internal RIAA equalization. Also the extreme headphone outputs became a little more powerful if I remember correctly. Plus new Pentaconn connector for balanced phones.

So the question might be also or more which feature set you need and what you are willed to pay just in case the selection process has also a budget component.

thanks for the detailed answer.
I have an ADI2 DAC (the second AKM version) in my hifi, but I'm considering to upgrade to a 2/4 precisely because I'd like to have analog in and digital out.
My understanding is that these devices share some (...) technology, so I'd expect them to sound _mostly_ the same.

As I'm in the mood for stupid questions, I may have another one: is a DPS-2 able to power TWO devices at the same time? e.g. both a DAC and a 2/4?

You're welcome.

If it is this alone, then you can choose between ADI-2 Pro FS R BE and ADI-2/4 Pro SE.

The product page tells: "Please note: only one output is usable at a time". So only one device per DPS-2.

Ah .. ok, has already been answered ;-)

just curious: can I connect two units and turn on just one?