How does a bass MIDI track 'hum'?? This confuced me; may have it routed to some kind of effect (?)
It should be a clean sound.

Anyway you can still fade it. Because it's MIDI the method is different.
Either the piano roll (using controller 11 in the selection box) and then draw it in the screen as an angled line going down,
OR use (I know it sounds weird):
Edit-Fill
Then Fill the highlited section with a control change 11 (or 7) fade out from whatever the initial setting is down to zero.
I've had luck both ways.

Another option is to right-click and generate the MIDI track to an audio track so the original 'Edit- gain change-fade' method works on the audio track (after muting the original MIDI track so it doesn't play at the same time!)

I do not work here, but the benefits are still awesome
Make your sound your own!