I'm going to re tube my 290 and am trying to save a few dollars by buying my tubes on ebay. I'm focusing on reputable buyers and new tubes, but am a bit concerned about bias. Do the tubes need to be biased to mesa's specs or am I okay with going with this approach? Do sellers like doug's or the tube store, etc. adjust or only sell tubes with mesa specs? I'm just not sure how the whole process works and don't want to blow my amp or something bad like that. Any input would be appreciated. Thanks.