Mesa Boogie has been into Bass Amps for a number of years and and we have thousands of customers using the Mesa 400 and 400+ amps. The most asked question we get from 400 and 400+ players is "will the JJ tubes work in my amp?" and "are there any tubes we can use other than 6L6's? The answer to the first question is yes, we have been tubing these amps for as long as Eurotubes has been in existence since 1998. These amps need a little warmer grade to get the bias up where it should be just like most Mesa amps.The answer to the second question is yes and no! The 400+ does not have physical room to use big bottle tubes so you’re stuck with 6L6's, however without a doubt the JJ 6L6GC's will pump out more power have deeper and tighter lows than any other 6L6 so we have a lot of very happy 400+ customers!The Mesa 400 is a different story, it has the physical room to not only run 6550's but it can also accommodate KT88's. You can get a very nice improvement in tone and power by simply using the JJ 6L6GC's but if you want to step it up a notch you can drop a sextet of JJ 6550's in which will make more power than the 6L6GC's and growl like and angry Werewolf when cranked! If you want to get as much clean power and headroom as possible and the deepest low end then drop in a sextet of the JJ KT88's and strap yourself down! This setup will make your 400 stay right on the tail of a 400+.Retube kits for these amps can be found here under Full Retube™ kits for Mesa Boogie Bass amps.