I have been having trouble with my Mesa 400+ since I bought it 2nd hand a few weeks back. I have now decided to buy a whole new set of valves for it but I am not entirely sure what valves I need for it. The Mesa Boogie branded valves are too expensive here in the UK so I am going to look elsewhere. Tubeshop.com seem to have good prices on matched pairs of valves but which ones do I need? I know I need 12 6L6's and four ECC83's but what about this bias business I just don't understand? Any help would be greatly appreciated. Thanks.