I just received my Bass 400 from ebay (in near mint shape, and with a cherry SUS-4 rack!), and it also came with the original operating instructions. Under "Power Tubes" it states: "The Bass 400 uses special Mesa STR 418 type 6550 power tubes... Do not replace these tubes with the larger metal base type 6550 such as Groove Tubes sell! First, they will not propertly fit into the hold down clamps; second, they are considerably more expensive; third and most important, they will fail constantly in normal use... Amplifier bias has been permanently set by us and should never need adjustment." On the schematic, it states: "V5-V10 use Mesa Type STR 418 6550 or 6L6." The amp came with 6L6's (STR 430), and the previous (and original) owner had also used STR 420's (also 6L6's). I'd like to run 6550's, but I am wondering what this "large metal base style 6550" bit is all about. Does anyone have any suggestions as to particular 6550's which would work well in a Mesa 400? Also, can the bias really be set such that it works well for both 6L6's and 6550's? Any comments or help are appreciated. Tom.