I have had a 2003 WT550 for a few years and have loved it, but recently I've been having some issues with getting the amp to turn on and stay on. It started like a finicky light switch where you have to wiggle it around the half-way point to get it to turn on, but now sometimes I'll have to try flipping the switch 20 times before it will turn on, if it does at all. Also, when it does turn on, sometimes it will just power off on its own. Has anyone experienced problems with this? From what I can tell the fuse is not blown, and sometimes when I turn the amp on I can hear an electrical hum coming from the head itself for a few moments. It's become unreliable because of this issue, and I would like try to fix it before buying a new head. Thanks in advance!