I've just spent half an hour searching for the fader controls the MIDI volume in KMix (but I suppose it's the same as in alsamixer).
Why couldn't I find it?
Because it's in the "input" section (as opposed to "output", where I've been looking) - and it's called "Synth".
The soundcard was a "Creative SBLive! Platinum [CT4760P]", in case that different models behave differently. who knows...