Left speaker has no bass anymore after removable cable mod.

Which headphone was it? Does the volume change on the left side if you wiggle the plug? Some pictures would help

Re solder the green wire. and what 3.5mm plug are you using? Did you make one out of the stock one from the m50?
 

Tbh, the solder joints do look suspect.  The ones on the jack look like cold solder joints.  Make sure your iron is hot enough and clean.  Tough to tell but any chance there's a small solder bridge on the driver?

If you are getting sound then I doubt it's your solder work. A short either has sound or doesn't, the same with a broken connection. Low volume and no bass means that your driver and more importantly, it's voice coil, has come too far out of the magnetic enclosure. There are several sweet spots where the voice coil is magnetically drawn even with the power off. It's function is to move in and out along the magnetic pole to produce sound. The problem happens when the voice coil gets out of this sweet spot, it doesn't even necessarily have to be all the way out, just out of the sweet spot and you will lose all bass and it will be very quiet, because it's running in a very inefficient position, like trying to listen to headphones without them touching your head, position is everything to an electro mechanical device like the driver.  This happened to me on my 880's when I was plucking hairs out of the driver and it's easy enough to fix.

1. Try air pressure. Gently blow on the speaker, you're trying to use air pressure to gently re-seat the voice coil. It's like fitting a ring on a metal finger.

2. If that fails, try the direct approach, use your fingers but be very gentle. Use cellophane office tape if you need to pull up on the driver or pop out a bubble or wrinkle.

Good luck.
 

blowing......That's why I mentioned it because it doesn't require disassembly. You're trying to gently push and pull on the driver using your mouth and air pressure, so it re-seats.
 

Impossible to say. If you hear the driver crinkle a little then it's working, if that didn't fix it though you either need to open it up or something else is the problem. If you could look at it you would be able to see immediately whether or not the driver had pushed the voicecoil out too far.
 

I know one way you can rule out a short or electrical problem though, stick a voltmeter on that driver, set it to ohms, and measure the resistance.