if i wanted to replace the 6L6 GC tubes with tubes of better quality than mesa's, what would i use? one of the reasons i bought this head was cause of the premanent bias of the tubes. since bias varies, how do i know what bias i have? i'm more concerned with reliability and dependability than sound. if it's a little different than what i'm used to, i can do some know twiddling. i know there's tube gurus out there, so thanks in advance for the help.