Lots of questions. Treat this as a troubleshooter's guide. None of these questions are designed to lay any blame.
1. Did you de-solder and re-solder? If you did, make sure you didn't switch the wires. The ground wire is always the same color and it's always on the left. Depending on the mismatch, crossing your wires can either (a) kill your bass, (b) put your drivers out of phase or (c) reduce everything to mono.
2. Did you liberate your drivers? If you did, make sure you don't have a crinkle in the driver. Also make sure you haven't pulled your voice coil out of its alignment around the magnet. Although nobody wants to touch the drivers, I'd do so if it were necessary to pushing the voice coil back. Crinkles in the driver can be fixed by smoothing table over the Mylar and GENTLY pulling the dent out.
3. Did you vent the diaphragm by popping holes in the driver back? If so, it's possible that some tiny speck of dust or debris settled inside the driver. This can be fixed by shaking and blowing. While it's hard to shake too much, it pays to use good judgment when air blowing the back of the driver.
4. Are you wires solid? Depending on the age of your headphones or any bumps that occurred along the way, you may want to check your connections.
5. Did you use the steam method or the hairdryer method in loosening the glue? It's possible for a tiny amount of water vapor to affect driver performance. The cure for this is to leave your headphones in a dry place for a few hours or days (depending on the condition).
6. Did you remove the grill cloth? If you did, it's possible a single strand of hair is laying on the driver. This can cause buzzing but it can also affect bass. Anything that prevents the entire diaphragm from moving freely will affect bass.
7. Did you change left and right so that you're listening to the cups backwards? In addition to affecting imaging, this can also affect your perception of bass. I wouldn't go so far as to say it causes you to hear things backwards but when you're used to the bass being more prominent on one side and that bass is not there (because the cups have been reversed) it can make you wonder where the bass went.