I know it's a tough pill but I have SE-310's with custom Westone molds and thought I was railroaded into staying with them. A long story short, I just stopped @ Best Buy and picked up the Klipsch I4's and think they blow away the Shures. I have been without the 310's for weeks totally pissed about the cable shorting and now am back in buisness. I can tune the EQ from my iPhone and use the Headroom total bithesd which just tightens the bass a little but the Klipsch's are only $80 and my Westone molds work luckily. I have my reciept for the Shures but the replacement was $80 and I just did a comparrison and can tell you I woun't be staying with Shure. I had problems like you with the older E2c's and read them the riot act and I can say I SHUREly will not be a customer of them anymore. Suck it up and move on is my suggestion to you and start enjoying the music again. Best of luck and Shure S**** bigtime!
I just mentioned this to someone else - while I'm confident I can do the rewiring (I'm an electrical engineer and solder things basically for a living on a much smaller scale) I wouldn't want to take the risk of messing up your earphones either.
If you decide to replace the se420s with something else and are basically going to trash them, let me know because I'd like to take a last ditch crack at fixing them (and would certainly throw you a few dollars for the chance).