Problem solved by Tinkering!

"Each time I load a 64bit plug-in (from SampleTank 3) everything works ok, except the affected track's volume slider in BIAB's mixer doesn't work. The track can be muted with the mute button, and the track's volume can be controlled with the volume control in the plug-in, and the overall volume can be controlled with the master volume slider in BIAB's mixer - but the track's volume slider might as well not be there."

The problem is in my use of the SampleTank 3 interface and its midi control commands. What works is the following:

Press the <Learn> button on the bottom left of the "Play" mode. Follow the screen directions, ie. select the track in question, then go to the BIAB mixer and slide the volume control that wasn't working. Presto, it now works as advertised.

I knew there had to be a fix. There usually is.

John E.