I have a Native Instruments Komplete Audio 6 (Mk 1 version) which is at the budget end of things, but its four outputs have worked just fine with Isadora 2.x on a Mac.
It's just a plug-n-play USB audio interface, equivalent to Ryan's Roland Rubix 24 or the Scarlett 2i4, and in the same price range. Designed and engineered in Germany :-)

Mark just a correction. I have the Roland Rubix 24. It is nothing special, but very much competing with the Scarlett 2i4.
My Roland also has 2 inputs and 4 outputs. I got it over the Scarlett because the outputs are all matching 1/4 inch (less adapters) and for recording it has a built in hardware compressor ( I am getting great sound recording for tutorials :)
Again, its in the same ball park as the Scarlett 2i4.. price was exactly the same when I was looking at them.
I have had no trouble setting up Quadraphonic output from Isadora.. it works well, and I have played with Quadraphonic mixing via linking my Daw with Isadora and OSC.

Are you going down the ASIO route I would personally just buy some Scarletts for that.

Whatever we choose, it needs to work with Windows Media Foundation so that Movie Player can get the sound where it needs to go. Routing sound from movie out to ASIO is too big a chunk to chew on in terms of getting this feature finished in a timely manner. Our goal is to allow users to use a standard multichannel audio interface to route any channel of a movie that has sound (or just a sound) to any output, and to do so on the fly interactively if desired. That will be a big step up from what we have now, and should address concerns that came up previously.

Best Wishes,
Mark

we are using Dante Virtual Soundcard, and it works perfectly.

I've got an implementation working well with macOS, but I need to get a sound device to test what I've programmed on Windows.

@fred said:

I don't know about your use case, so maybe it is not a good investment for you. For me it's great.Edit: Aha, now I see, if it's only for testing multichannel output then grab any class compliant card. For a Soundcard to use for performances and installations as part of a kit, I would go with RME

 OK! That also is super helpful @Fred. I mean, I want to work with something "real" in terms of what's out there. But to have to spend 3 x 850€ is a pretty big investment for us. I may still pop for one of these since you and other recommend it. But it would be cool if we could keep the costs down in terms of testing.

 The solution for Win was RME

 OK, that is important to hear. Thank you.

But I know there are a lot of trouble with Win for 8 channels (for 2 is is running without an issue) . The solution for Win was RME ....

Being able to mix, make submixes, remote re-route channels, eq and compression in hardware, osc control and midi control of ALL functions, the reliability, and the compact size make RME worth it for me (I have even used it as a mixing desk with an old iPad to control it). I buy from a place that has a 3 year warranty on all products and now mine is 6 years old, and I use it regularly, every show I have that has sound and it still gets driver and software updates quite fast (some brands will support a few versions of Mac OS and dump you in legacy land). So now I've paid €3 per week for something that has outdone itself for years and would not hesitate to recommend it. 

I don't know about your use case, so maybe it is not a good investment for you. For me it's great.

Edit: Aha, now I see, if it's only for testing multichannel output then grab any class compliant card. For a Soundcard to use for performances and installations as part of a kit, I would go with RME.

Wow €850 for the RME UCX. Do you really feel the Scarlett won't allow us to do meaningful testing? (You said it was buggy.) I'm going to have to buy mulitple units so the team can test and we need to watch every penny here.

I mean, I have a MOTU Traveller but it's ancient and I don't feel good about testing with it.

In addition, does anyone have feelings about the Roland Octo-Capture. @DusX already has one of these and has used it for multichannel stuff, and it's half the price of the RME. Just curious if anyone else has somethign to say about it.

Best Wishes,
Mark

There are other models with more channels like the UFX II and UFX +, still all current.
The only downfall of the UC model is that output 7&8 are shared with the headphones output so unbalanced when used in line mode, but the ADAT gets you an extra port for 8 more channels.
The matrixing and mixer and real time hardware effects on the RME stuff is amazing and the remote capability is awesome. Still I would be leaning toward Dante based stuff, like the rio or tio if it was not for wanting mulitchannel audio output in windows.

go for the ucx

 Really? isn't the RME UCX a USB 2.0 / FireWire 400 model?

Are you saying there is no Dante VS for windows?!

But there is...

https://www.audinate.com/produ...

Thank you for this input. Since it seems RME is a powerful, professional system -- and that @Fred and @Maximortal disagree about Scarlett -- I guess I would try to acquire an RME interface. I would like to get the least expensive 8 channel system I can... preferably USB over FireWire (a lot of the RME devices seem to be the latter.) 

Given those specs, can you all make a recommendation?

Best Wishes,
Mark

On my MacBookPro I have a Dante Virtual Sound Card (https://www.audinate.com/produ...) you pay $ 30 for it. I can send multichannel audio embedded in a Video from Isadora to a RIO.

Best Michel

I have been using a Dante a lot and there is a Yamaha TIO and RIO that are really good and flexible but at the moment Isadora cannot send a mulit-channel audio with video to these, if it could, because of the flexibility of Dante I would suggest that.

as mentioned before a digital mixer can be also a very good option. An x32 can be found around 1300 € and you got a mixer an Audio interface and a control interface all in one.