I was practicing with one of my bands a few years ago, with my Mesa Boogie Bass 400 (non plus) head, when the speaker cable blew out the back jack. My instinct was to power it down, so I spun around and lunged for the power button, but ended up hitting Standby instead. There was a small blue electrical spark and Standby ended up fusing into place. At the time I was quite low on funds and had just purchased an Eden WT-550, and was unaware of the resale value on the 400 so I quietly put it in its case and stored it. Fast forward to now, I'm playing a lot more and am interested in getting it back working again, and wanted to ask for input from anyone with experience repairing tube amps. At this point the amp will power on, and all of the power and pre-amp tubes glow. Unfortunately, since Standby is fused into the Off position the amp just can't come out of standby. I've taken the bottom cover off and looked at the area around the Standby switch and there does not appear to be any scoring or burn marks around the switch. Does this sound, at "first glance" like I just need to replace the Standby switch? How likely is it that the initial scenario, speaker jack flys out of the head while the amp is being played through and then Standby is hit, could have caused serious damage? Which components in particular could have been damaged? Lastly, just asking because it has been forever since I used this head. Are the paired speaker outputs independent or parallel? Specifically, if I'm running 2 8ohm cabs, do I use the 2 4 ohm outputs, or the 2 8 ohm outputs. I'm assuming the 4 ohm, but I'd hate to break this thing once I get it fixed.... In closing, I'd appreciate any advice as, due to my lack of experience with this stuff, I'm going to have to pay someone a few bucks to do this and it would be great to walk into that armed with at least some knowledge beforehand.