It occurred to me that Rui Paulo wrote in gmane.os.netbsd.devel.kernel:
> Hi,
> I have an eap 1370 and I'm trying to figure out how to make 
> ``mixerctl <var>++'' work correctly:
>
> % mixerctl -w inputs.dac++                                                  [~]
> inputs.dac: 200,200 -> 200,200
> % mixerctl -w inputs.dac+=8                                              [~]
> inputs.dac: 200,200 -> 208,208
> % mixerctl -w inputs.dac--                                                  [~]
> inputs.dac: 208,208 -> 200,200

It looks like this particular mixer input has a delta (min. step size) of 8,
but the driver mis-reports it as having a delta of 8. See man mixerctl.

On my (auich) soundcard, mixerctl -w inputs.dac++ works as expected:

% mixerctl -w inputs.dac++
inputs.dac: 199,199 -> 207,207
% mixerctl -w inputs.dac--
inputs.dac: 207,207 -> 199,199

On my system, I also get

% mixerctl -v inputs.dac
inputs.dac=199,199 volume delta=8

I think the eap driver misreports this 'delta'.