Greetings! I just got a brand new MB 400+ about a month ago-tried it out at the dealers before I got -the dealer changed the four AX tubes in the back 'cos it was making a little noise. The noise disappeared. My question is: The amp is making noise again even I am not plugged into it & the amp is off standby. The noise sounds like a slight crackling sound that I would definitely not want there if I was recording but once I start playing through it-it seems to sound fine. Have I run into some more "bad tubes"? Any of this sound familiar to you veteran 400+ owners? Any suggestions will be greatly appreciated!