Sup guys! Recently I feel like I've been having to crank the gain on my amp a lot more than normal to get it "peaking" where it normally does. I'm thinking it's a preamp tube(s) issue, so what would be the best way to check for a bad tube? Also, if it is a tube, what would you all recommend as a replacement? I like my 4 Pro to be as clean as possible because I just use distortion pedals for my sound. Thanks!