So I sold my 370 about a year ago and have been missing it ever since. A really clean one popped up on Craigslist and I traded an Ampeg combo I had for it. Opened it up to change one of the resistors to a variable trimmer so I can get more clean headroom out of it. While in there, I noticed this red button. Anyone know what this is? Probably something basic, but I never noticed it in my old one. Thanks!