I just bought this amp for $450 on eBay. http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7400624128&rd=1&sspagename=STRK:MEWN:IT&rd=1 I think I got a good deal but I am thinking that maybe I should just retube it from the git-go to start with as good a sound as I can get. This is my first tube amp. My questions are: how do I tell if the tubes are in need of replacing? What tubes do you recommend? is it hard to do myself (I'm fairly handy with basic electronics). What is bias adjustment and should I get a pro to do that for me? Thanks!