Hi Sascha, I'm still waiting to receive the ADI-2 Pro I've ordered a few days ago, I haven't tried it it.
But I have received my new UFX in the meantime, and it adds to my UCX... don't know about the UFX+, no experience on it, and it's not on my path, no need for such a load of functions and channels in my application.
But I've been using the UCX for the past two year (or three, maybe), driving sound systems you can't imagine how big and how expensive, and the volume control of the UCX is like no other audio interface i've had a chance to try. I wouldn't have had any reason to add other interfaces, if not for having new colleagues who need their own interface, and for the need to drive long AES/EBU lines (which isn't the UCX forte, obviously).
To me, the solution until now has been to mate the UCX to an ADI-4 DD, which never failed me and had the added value of a DB25 connector carrying four discrete AES/EBU outputs out of a very small device.
Is the DSP of your active loudspeaker provided with only a digital input? If this is the case, then the ADI-2 Pro won't do it, unless you opt for the loopback suggested by MC, which is indeed a very practical and easy work-around to an architecture that's been designed with a different (and very flexible) purpose in mind. It won't do in my case (we're counting bits in my application), but will do in my home setting (which is why I ended up buying it anyway).
Can't say what's best or what's worse, sorry, can't help you on this. As far as I'm concerned, though, I've had ample proof that the inherent quality of modern converters is very seldom behind an unsatisfactory sonic appraisal, as differences between "this" converter and "that" converter are very hard to pinpoint if no words are truncated, or levels aren't dithered. Which is why (and it's commendable, indeed) the ADI-2 Pro has been designed WITHOUT an output level control in the digital domain when performing a format conversion.
Again, not sure if that's your case, but converters driving outputs at -20dB or worse, lower levels, can hardly sound good, unless a considerable amount of dithering is performed in the background (which, before today's very efficient DSP processing, was a clear enemy of low latency). As far as my experience with the interface you're mentioning (and I played just yesterday a system for a demonstration to a client who came equipped with that exact interface of his very own kit), I can't honestly find anything bad, or wrong with its conversion (if all the word length is used, without being chopped by a debatable gain-setting sequence in the audio chain).
If I was concerned with the smoothest possible volume control, I believe the loopback function applied as MC suggests is a great solution in terms of quality, and most likely quite better than any digital-domain-only volume-changing instance, that's clear as daylight, especially at the low-lower-lowest levels that are typical of home applications (in which you listen to music also at night, early morning, or when your partner's still asleep, often at -30, -40 or worse attenuation).
I would have not discarded the ADI-2 Pro if only I wouldn't have had to run it into a signal chain that's designed to make do with one less conversion, or two, from mixer to power amps of concert sound systems; in simple audio terms, though, no doubt it sounds as good, or even better, than any digital "attenuation" performed without some serious dithering, again an area in which RME devices made all the difference in the world compared to other-brand AES/EBU-equipped audio interfaces (of which did not exist many different devices until very recent times).
I'll let you know how it turns out, when I can finally get the ADI-2 Pro in my kit. It's meant to replace my previous 2-piece converter + heaphone amplifier set, with a great saving in terms of footprint, bulk and weight (CEC DA-53 and SPL Phonitor first version). As I often play along much of what I listen to, I expect my Babyface Pro will come in handy for my guitar and computer DAW low-latency job fed into the ADI-2 Pro.
If you don't need to run 50+ meters of AES/EBU lines, though, I can't speak good enough of UCX in terms of audio quality, if you really wanna replace your FF400. I was stubbornly set to FW in my mind, and FW turned out to be the least used option onboard my UCX, never ever found USB unable to keep with my 2 or 4 channels of audio feeds. With the length of a home or studio environment, the unbalanced, 75-Ohm SPDIF output pigtailed unto a short cinch-to-XLR adapter never itched even once in its lifetime, in my experience, so I believe 20m at 96k with the right cable is no sweat at all to UCX, if you like it compact and half-space.
Hope this helps you look around in all possible directions, it's just my humble opinion, but can't see much call for a fortress like UFX+ if your channel count stops at... two, and even then, UCX has as a lot onboard you won't ever need. MC's hint makes the most sense of all options (if you just don't have my limitations), in my own view.
