Why would this gentleman need a Dac based streamer sucn a Node when the Hegel has a good dac built in?The digital volume control applies to the digital outputs of the LUMIN products, and the LUMIN U1 and U1 Mini do not have analog outputs. What I confirmed last night was a LUMIN feeding S/PDIF out to an external DAC, with the digital volume control engaged.
In other words, if OP's guests AirPlay to a LUMIN, it will volume limit playback via digital out to any DAC (i.e. the Hegel H160).
For EQ, the new miniDSP SHD devices also accept AirPlay and have built-in DSP options. I don't recall if there is a preset volume setting you can control though, for when AirPlay would kick in. However the UX is less polished than some other devices which might create some annoyances for you in exchange.
Unfortunately I can't think of any other AirPlay-compatible devices at the moment that will override the source's volume setting.