To my best knowledge a DAW/application can only load one ASIO driver, in this case either from Focusrite or from ADI-2 DAC.

Even if something like this would exist I wouldn't recommend it. Why? Because then you have to route always through the DAW with the full RTL (round trip latency) which can become quite high. Especially if the ASIO drivers of other vendors (compared to RMEs excellent drivers) are not so well written. Another question is whether latency compensation would work well enough when recording to a backing track.

I think the best solution would be to say "lessons learned, I bought my recording interface too small" and to plan for a better setup.

I see three possible options, a little bit more than €1000, but which are close to that, not too expensive and defiinitively a much better solution for the money.

a) sell Focusrite and buy a much better RME recording interface for the money, e.g. UCX II.
https://forum.rme-audio.de/viewtopic.php?id=33222
https://www.rme-audio.de/fireface-ucx-ii.html

By this you get something which is already superior in every aspect compared to a Focusrite Solo. It doesn't have all the features of an ADI-2 DAC, but the phones outputs are already of high quality.

b) If you definitively want the ADI-2 DAC, then get rid of the Focusrite solo and then get a combination of RME recording interface and ADI-2 DAC FS and integrate it through ADAT or SPDIF like described here: https://www.tonstudio-forum.de/blog/ind … our-Setup/

This list of RME USB/FW/TB/ might be useful to you to get an overview (price/features/technical specs):
Blog article: https://www.tonstudio-forum.de/blog/ind … B-MADIfac/
Direct link to the Excel: https://www.tonstudio-forum.de/index.ph … -04b-xlsx/

c) if money is no hindering point I would go for a combination of UCX II and ADI-2 Pro FS
Because then you can use the UCX II AES port to connect the ADI-2 Pro FS and still have the ADAT port free for future extensions like e.g. connecting an 8-port preamp or ad/da converter or to connect to your HiFi and to use the PC as music player or to use the HiFi as 2nd pair of speakers (TotalMix FX: "Main Out B") when mixing/mastering.
The following blog article gives you an overview about the differences of ADI-2 DAC / Pro:
https://www.tonstudio-forum.de/blog/ind … ses-EN-DE/
By this you get also a SRC (sample rate converter) into your setup which can be very useful when connecting devices, that can only act as clock master or have a fix clock setting which might be different of the sample rate that you want to use in your recording projects (e.g. DAT recorder to archive old recorded tracks or a guitar modeler).

Well, I've used UFX and UFX+ alone long enough and the converters from the UCX II aren't any worse either.

This would give you an excellent basis for your setup in terms of quality and features that cannot be topped by the competition.

At this point I would like to emphasize that the UCX II really got a lot from the flagship interfaces (UFX II and UFX+), for example
- high quality preamps with 75dB gain
- DURec for standalone and backup recordings on USB disk/ssd/stick
- Clear color display, also ideal for use in standalone mode
- a complete FX section (like the predecessor)
- very good headphone output
- AES I/O has been added

So you have everything you need and still have certain reserves, so that you don't have to buy something new after a while.

You can buy an ADI-2 DAC or Pro FS at some point later and integrate it into this setup in order to enjoy further sophisticated RME features.

> Edit: it seems like as long as I’m not using power hungry planar headphones, the UCX II should be good enough
> by itself. Or I assume I can add an amp if necessary.

Which phones do you mean ? I am using planar headphones and wrote a review about Audeze LCD-2, LCD-X and LCD-3 (which I own). There was absolutely no issue driving these phones with UFX+, ADI-2 Pro FS, Octamic XTC.
To have planar headphone doesn't mean, that you have headphones with high impendance or inefficient drivers ..

Last but not least .. if you are also doing recordings etc .. then it's more important to get a good recording interface with a digital mixer first, so to say UCX II with TotalMix FX.
The ADI-2 DAC pro is 1st of all an USB DAC without TotalMix FX.
The ADI-2 Pro has some more routing features, but also this is much more static compared to the capabilities of TM FX.

Therfore: good recording interface 1st and if you have the money and interest: ADI-2 DAC/Pro on top.

Further information incl. links to excellent RME training videos on youtube which are on the point and do not waste time you can get from this posting: https://forum.rme-audio.de/viewtopic.php?id=34394

Well need .. I personally like different features like
- dynamic loudness, it gives me much value when listening at low volumes when working in HO
- the ADI-2 Pro acts also like a monitor controller in my setup not to hurt my ears if I would accidently turn the faders to 0dB in TM FX. Ok, I could also buy attenuators, but with such a setup I do not need those (have anyway no room to pluh them as the monitors stand at the walls on my desk).
- ADI-2 Pro provides e.g. slow ramp-up of volume when plugging phones or switching between phones and line out

But this you can partially solve also in a different way which is not so cost intensive by using attenuators.

You want to connect a shelf speaker, I think this refers to a small active monitor, to the UCX II through AES ?

I see some disadvantages

1. AES uses 1 cable and audio for left / right channel is part of the header in the audio packet.
2. Not every monitor offers a built-in splitter to carry this digital signal over to the 2nd active monitor.
    In such a case you need an AES splitter which costs around €419
    https://www.thomann.de/de/mutec_mc_11_a … verter.htm
    Cheap y-cables also exist, but I am not sure whether it would bring more problems than benefits.
3. Then you have the D/A conversion in the active monitor, the question is how good the d/a conversion there actually is
4. if you accidently turn the faders to 0dB then the monitors play at maximum volume, depending on the quality of the
    device it will survive this, your ears possibly not.
    So it's a little bit easier to connect active monitors analog and to solve such level mismateches by the use of attenuators.
    https://forum.rme-audio.de/viewtopic.php?id=25399
    These two attenuators are switchable:
       https://www.thomann.de/gb/jts_ma_123.htm (-10, -20, -30 dB 22-20kHz)                     €22)
       https://www.thomann.de/de/shure_a15as.htm (-15/-20/-25 dB 20-20kHz +/- 0.5dB      €67)
    For XLR (balanced) and you place them best in front of the connected pre/power amp.

On the other hand there are active monitors available with speaker management software and automatic calibration.
But they are of course not cheap, like e.g. from Genelec https://www.genelec.com/glm

See also this thread: https://forum.rme-audio.de/viewtopic.ph … 48#p181548