I have an older 400+ that i've used for the past 8 years. I absolutely love it's tone but it has one thing that has annoyed me for years. Every time you switch it off of standby it makes a series of low volume pops as well as some hiss/hum. After about 5 minutes of being on it no longer makes the pops so it's not a huge issue. It is mainly used in a church setting however and therefore I can't turn it onto standby during the sermon as when We go back up to play the last song if i switch it off of standby it pops for the next 5 minutes. I just replaced all the tubes Pre/Post with a set of JJ's from eurotubes hoping it would solve the problem (it needed new tubes anyway) however the sound is still there. The sound is not affected by either the master or pre-amp volumes and will will occur even when everything is set to 0. I made a short video showing what it does at YouTube - Mesa Bass 400+. Any body got any ideas on what it could be? I'm pretty handy with a soldering iron (have made several pedal on my own etc) and unfortunately there is not tech in my state (Missouri) I'd trust with a tube amp like this.