My FLY bass amp has a knob for volume and for gain. The manual says that the gain is for distortion and overdrive. BUT, when I turn it off, the volume is also off, even if i have the other volume on/full. What's the point of having a distortion/gain knob when you can't turn it off? 
I like distortion but obviously it doesn't work with all songs.
Please if someone knows more about it could you explain if I should have just bought something else or there's smth wrong with this. 
Also, when I use my headphones there's almost no distortion compared to the amp sound. (using good sennheiser hd pro headphones)

thanks in advance

And here's the link to the amp on the official website
filiosp1234  this happens on many devices (effect pedals and amps) that have both a gain and volume control.  The gain knob creates distortion by increasing the signal level going into a circuit that saturates (thus causing distortion).  If the gain knob is set to 0, then the signal level is 0, and you get no signal out.  

Just set the gain knob very low and the volume very high to get the cleanest sound that the amp can provide.  How loud are you trying to play?  It's only a 5W amp with a 3" speaker so there's not going to be a lot of clean headroom.  You'll likely need a higher power amp with a larger speaker to get more clean headroom.

Also, doesn't that amp have both a clean and OD channel?  Just use the clean channel?
8Len8  Well these are the controls https://www.blackstaramps.com/img/ranges/fly-bass/fly-bass-top.jpg . You mean turning off the OD button for the clean channel? Still does have some distortion with it, but not as much. But yeah I try messing a bit with it having low gain and highest volume.
I was mostly wondering if I could get it really clean, but I should have considered this more when buying the thing :p. Luckily my headphones are great and I'll probably get a good amp when I can play well enough in some years time.
But then again, it's probably hard to get a good clean sound with a small amp such as this as opposed to high quality Sennheiser studio headphones. (HD 380 pro)
Thanks for the reply tho