so as you know, i bought a used mesa 400+. when i recieved it in the mail, one of the tubes didn't light up like the others. i'm assuming it was blown, so i took it and its match out. it wasn't a mesa labelled tube...it was a sovtek. anyway...the sound wasn't good at all. i couldn't get any kind of close to a clean tone. the amp is a 95, and i'm assuming all the tubes in it are stock. so i ordered some jj6l6gc's and jj 12ax7's that i just got today. anyway...my question is this: since mesa amps have a fixed bias, am i alright with just putting the new tubes in myself? or should i take the amp to a tech to have it checked out? nothing seems to be wrong with it, other than the tubes sound bad because they're so old. everything seems to be solid. will it be alright to just install the new ones myself? the reason i ask, is because i don't want to spend too much money here (i haven't deciced if i'm keeping it yet), and the closest certified tech is about an hour and a half away.