I ordered a Behringer ADA 8200. Bit of overkill with it's 8 channels, but it was cheapest microphone -> ADAT interface I could find. Sadly I couldn't get HOLMimpulse to work, which used to work before, but I could make measurements with REW.
I set the ADA to 48khz master clock mode and configured my RME card to use this as clock source. You can easily create a single WDM soundcard for each 8 channels in the MADIface settings, you just have to use the first two to in analog world to make the applications happy (L/R). With ASIO, you can do whatever you want anyway.
Long story short: Wow. I had so many phase issues by using two USB soundcards. Even when the bass was okay, in the upper registers stuff was all over the place. This is not a subtle difference. Totally worth it to me. In hindsight, I should've gotten the Fireface UCX instead of the Digiface. But I wanted to run external DACs, because of the reviews here. Speaking of which - @amrim I would be very happy if you could review the RME Fireface UCX.
You can basically do everything with this thing. It's capable of multichannel playback and you can do measurements. Maybe a bit overkill for people only interested in stereo, but I would like to know how it compares to the latest ADI DACs, which we know are excellent.