I notice that my BG250 115 combo hums when the gain is set for example at 3/4 and the volume is set at 50%.
This occurs when no guitar lead is plugged in and occurs at different venues. It is not too loud to make it a problem using the amp but I wonder if this is normal. It increases if I turn on the tube drive and turn it up to say 40%
This is completely normal.
Here is the input gain specs: gain range -96 to 36 dB. It means that your input gain can really boost the IN LEVEL to very high values - the 3/4 position is a very high setting if you consider the average level of a "normal" bass, even a passive one.
In other words, the chance to have the gain knob set at this value is quite low. Unless your bass has very weak passive pickups.
Without a bass connected, putting the gain at this high value will just raise the noise floor. If you add the EQ boost + TubeTone + TonePrint you can of course get an even louder hum and hiss. But that is not the way it should be used.
The input gain is not a volume - It must be used to precisely match the output level of your bass to the input stage of the amp in order to get the best signal to noise ratio without distortion. . It is a calibration control. The only volume control in the amp is the MASTER knob.
If the Input gain is correctly set, the general output should have a very nice S/N ratio, and no hum should be noticeable.