Just adding yet another method (method #4 ?) I now have working, to feed Spotify/Apple Music/Deezer/Tidal through HQPlayer.
Wouldn’t be possible without Jussi’s help.
And this method fully supports the holy grail - automatic sample rate switching 
I am feeding Apple Music Hi-Res Lossless, Qobuz, Tidal (all varied sample rates) from my iPaid Air 4 on my desk. Using their official apps, not a 3rd party app.
This method supports any SPDIF source (CD player, streamer, PC with built-in SPDIF output, USB to SPFIF converter).
It is using an RME ADI-2 FS DAC which features duplex / bi-directional USB.
So you hookup your SPDIF source (TOSlink , digital coaxial RCA) to the ADI-2 and it essentially works as an SPDIF to USB converter.
You also need a HQP OS / NAA OS to serve as the NAA input (HQPlayer will see this as ADI-2 SPDIF input). You don’t need HQP Embedded license for this device since you are only using the NAA part.
This will be obvious to Jussi and anyone else that has this working - there are some manual steps required for HQPlayer to ‘see’ and communicate your particular ADI-2 serial number.
I will leave those steps out for now but of course can share if anyone uses this input method.
The most important thing is that with an RME ADI-2 connected to HQP OS / NAA OS and SPDIF source , you can have auto sample rate switching !
The advantage over the BlackHole method is obviously auto sample rate switching but also you can use a streamer (eg Bluesound Node or whatever) instead of having a PC/Mac inside your living room.
Jussi has this working with ADI-2 Pro FS and I have it working with ADI-2 DAC FS. I picked up a used version 1 model for cheap (the original AKM 4490 chip) to use as input.
It probably won’t work with the cheaper RME Babyface Pro FS (different USB interface, different driver code). Jussi has spent coding time getting the ADI-2 family devices to work.
Of the 4 methods I have outlined in this thread, this is probably my favourite now.
Hope this helps !