Volume keys with an external USB DAC

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Hello,

I'm using PA 3 with my HTC 10 (internal Hi-Res DAC), to which sometimes I connect an external USB DAC/AMP (iBasso D-Zero mkii).

Everything seems to work fine: the external DAC device is recognized by PA, with the correct parameters (24 bit - 96kHz).

There is one thing that isn't clear to me: when using the external DAC, the volume can still be managed by the volume up/down keys of my smartphone,

 while I expect that this shouldn't happens, since the digital output is passed to the external device that should do all the work (DAC + AMP + Volume).

With my other players (Neutron and UAPP), when using an external DAC, the volumes keys on the smartphone do nothing, the volume can only be managed by the external device. 

Any ideas?

Thanks!

Stefano