Change audio volume on the right device

I'm trying to change the master output volume of my USB DAC (PCM 2704). When doing

aplay -L

I get:

null
    Discard all samples (playback) or generate zero samples (capture)
default:CARD=ALSA
    bcm2835 ALSA, bcm2835 ALSA
    Default Audio Device
sysdefault:CARD=ALSA
    bcm2835 ALSA, bcm2835 ALSA
    Default Audio Device
default:CARD=DAC
    USB Audio DAC, USB Audio
    Default Audio Device
sysdefault:CARD=DAC
    USB Audio DAC, USB Audio
    Default Audio Device
front:CARD=DAC,DEV=0
    USB Audio DAC, USB Audio
    Front speakers
...

but when trying amixer, I only have:

Simple mixer control 'PCM',0
  Capabilities: pvolume pvolume-joined pswitch pswitch-joined
  Playback channels: Mono
  Limits: Playback -10239 - 400
  Mono: Playback 0 [96%] [0.00dB] [on]

How to change volume of the USB DAC? And first, how to select the USB DAC device in amixer -c ? I don't know if amixer -c 0, amixer -c 1, ... (None of them seemed to work)