How  far can you turn this up wihtout blowing it up!? Code 50

Ive got a main  Heavy Metal Tone I use all the time. The volume in the preset is set to  1.8. I have it that low so it doesnt blow my Clean sound away when I click between the two. 

I was playing with a drummer the other day and I had the master volume up to maybe close to half and I was worried I was going to blow the speaker.

So How  far up is safe?