Bluetooth Headset volume too low (only in arch)

VMG's answer is subtly wrong; it will technically work, but it will disable all other plugins than a2dp, meaning bluetooth keyboards/mice/gamepads/etc will stop working, when the only plugin causing issues seems to be one called avrcp.

Edit

/lib/systemd/system/bluetooth.service

and change

ExecStart=/usr/lib/bluetooth/bluetoothd

to

ExecStart=/usr/lib/bluetooth/bluetoothd --noplugin=avrcp

and run

sudo systemctl daemon-reload
sudo systemctl restart bluetooth

My fix on kubuntu:

sudo nano /lib/systemd/system/bluetooth.service

Change

ExecStart=/usr/lib/bluetooth/bluetoothd

to

ExecStart=/usr/lib/bluetooth/bluetoothd --plugin=a2dp

then

sudo systemctl daemon-reload
sudo systemctl restart bluetooth

The effect should be immediate!

Removing AVRCP did not work for me with my WF-1000XM3. What worked was in fact the opposite. I used AVRCP to send volume up commands directly. I used D-Bus to interact with BlueZ following this answer.

In my case the full command was:

dbus-send --print-reply --system --dest=org.bluez /org/bluez/hci0/dev_<mac_address_of_headphones> org.bluez.MediaControl1.VolumeUp