Many thanks for the contributions.
dknk - my conclusion is similar, but I find my disappointment is aimed at Volumio - it seems to me that Volumio is only doing MQA passthrough, not decode - so the ifi DAC is not getting a decoded file and is therefore not unfolding it.
I have a Raspberry Pi 4 running Volumio
I have this outputting audio via USB to an iFi Zen DAC - which is an MQA authenticated renderer.
I have Tidal (HiFi subcription) running on an iPad
play track on Tidal app and switch output to Volumio in Tidal connect mode.
Effect - music starts playing - volumio registers that it’s better that CD quality, but the DAC does not show MQA detected.
play same track on Tidal app but connect iPad directly to the DAC by USB.
Effect - music starts playing - volumio is not involved - and the DAC MQA lights up to show MQA. However - using 4 USB cables and WiFi means that the track kept pausing. Need to do more testing to see if this was the cables or the WiFi - downloading the track seemed to help, which obviously suggests the WiFi
play same track - via Volumio - direct and with no involvement from Tidal or iPad.
Effect - music starts playing and no MQA light.
as a control - use Roon. This involves the same hardware - but with the Raspberry Pi running Roon Bridge under Linux and the iPad running the Roon app to call up a Tidal track via its own app. This time the Roon app is the decoder and it handles all the networking via its own proprietary protocol.
Effect - music plays and the DAC light shows MQA - plus the Roon app gives a full breakdown of the playback chain and reports exactly what is going on at each stage.
So - the Tidal app on the iPad is acting as decoder (this is not the case for the Tidal app on Android, as Android audio output on USB is different - I believe like Direct Show in Windows) but the Volumio app is not - whether direct or via Tidal connect.
Ie - in order to properly unfold MQA using Volumio - with the convenience of Tidal connect - you need a full fat MQA device which is capable of acting as decoder and renderer - these are expensive to the point that using a Raspberry Pi becomes non sensical.
My aim is to make no compromises on quality within reason, but maintain convenience in the sense of using a tablet for interaction.
Previously I had ruled out Volumio because I didn’t like the interface or search (which I still don’t) and was really impressed with the news about Tidal Connect. That seemed to solve all my problems.
But - it doesn’t.
I’ll still need to cough up for a Roon licence. Or - get a better quality long USB extension