I've already done a couple of TB searches on this, but still have a few questions. The 400 is best equipped with six 6550s The 400+ runs twelve 6L6s The 400 has a six band EQ The 400+ has a 7 band EQ The difference in power??? Why would Mesa make the switch from only six 6550s to twelve 6L6s? Wouldn't this increase weight as well as reduce the reliability (in that one bad tube drags down the rest)? Is there some sonic difference in having more low-power tubes? Is it possible to replace the 6L6s in the 400+ with 6550s to get more power? Are the 6550s more expensive and do they not last as long? I am very confused.