Sorry for late reply…
I am using a Raspberry Pi 2 with Hifiberry Amp attached.
Mixer type: None
Despite not seeing the MaxVolume setting anymore it still is in effect, I had it at 70 before the option disappeared in the UI, I still can’t raise the volume beyond that.
Edit: I decided to upgrade Volumio and factory reset the device after this. It brought the Max Volume setting back (and changed the reported Mixer setting to Hardware), but on the other hand the volume is now quite muted compared to previously, and if I turn up the volume above 70 the music starts stuttering. I have to look into this.
Edit 2: I reflashed my SD card with Volumio and things seem to work much better now. It appears to me I keep having to do this more or less each time I use the Update feature of Volumio, as I run into all kinds of weird behavior.