I've not had any luck using alsamixer with my IC-705 in Mint. The only thing alsamixer identifies is the HDMI sound. Yes, I've tried hitting F6 to select anything different - my Linux install does not see the IC-705 as an audio device.
I did install the PulseAudio volume manager...
sudo apt install pavucontrol
I can adjust both the input and output with that. I had to take mine way down to about 13% for about 1/3 deflection on the IC-705's ALC meter which will give you clean output and not overdrive.
Regards,
Joel - W4JBB