Ok, I think I've managed to work it out.

There doesn't appear to be a relationship between the Hardware Input slider in TotalMix and Windows volume.

Only the Gain knob in TotalMix seems to affect the Windows volume.

Previously, when the gain was higher, I was being deafened in TotalMix, but could only hear a whisper in Windows.

The solution was:
1. Put the TotalMix Hardware Input slider down to the very bottom (no sound in headphones)
2. Turn the TotalMix Gain up, until the Mic volume shows it's high enough in Windows Sound Settings
3. Slowly increase the Hardware Input slider so you can hear the Mic in your headphones

Hope this helps someone else!

Hi Daniel,

Other than the Gain knob, which you state is 'before AD conversion', is there any other way in TotalMix to increase the volume of WDM Analog (1+2)? i.e. after AD conversion?

Ok, understood thanks Daniel.

The reason I was asking is that Ableton (ASIO) picks up the input signal with a much higher gain than Windows (WDM).  I can't do much about Windows though, as it's already at 100% volume.