I have the oppertunity to get a used 400+. supposidly every works and sounds great. the guy said that all of the power tubes are Mesa 6L6's with a "red" rating except for the rear center pair which are rated "yellow" and one of them is a Mesa 5881. The tube socket and the grid resistor under the lone 5881 looks burned up. I owned a mesa m-pulse 600, so i have not had any experience with power tubes. i was under the impression that you would wanna put all the same type of tubes in the amp, all 6L6, or 5881, not mix match. also is that area thats burned up any cause for concern? thanks.