There was a problem I had recently with MIDI volume on Windows 10 and Sound Blaster Audigy with kxProject drivers.

I managed to solve it before posting, but I thought I'd share the solution if anyone else experience the same bug.

The MIDI sound volume, irrespective of the SoundFont used, is by default very loud compared to wave sound. That wouldn't be a big problem by itself, but the "Synth" slider in Windows 10 mixer doesn't work well for me: most instruments are adjusted properly, but some always stay at full volume with ridiculously sounding effect.

I don't know the reason of this behavior, but I found a workaround:

Play some MIDI file in a loop so you have real-time feedback.
Put the useless Synth slider in Windows 10 mixer to max.
Open the DSP settings (right click on kX icon in tray, select kX DSP).
A block diagram of soundcard's (virtual) internals will appear.
The interesting blocks are: FXBusX, FXMix2 and xrouting.

Delete all the routes coming from FXBusX's outputs FXBus12 and FXBus13. MIDI will go silent at this point.
Right click on empty space and add new St Vol block.
Connect FXBus12 and FXBus13 to St Vol's inputs inl and inr, respectively.
Connect St Vol's outputs to xrouting inputs SynthL and SynthR. MIDI should be back but with no reverb.
Connect St Vol's outputs also to FXMix2 inputs in1_l and in1_r. Reverb should be back now.
Double click St Vol. Adjust the slider to your liking. In my case 33% sounds just about right.

Save the settings to some file so you won't have to redo this.

Here's the diagram. Note the additional St Vol block on the left.