Thanks for the picture. You're right, I can't see well enough to evaluate the quality of the solder joints, but I suggest you try to touch up anything that seems off and see if that gets things working. It really sounds like the only problem is with the emitter part of the circuit, so if you can get them to turn on, the 3pi will likely be back to full functionality. You might find it helpful to look at the emitters using the display of a cheap digital camera, such as those commonly found on cell phones. If the camera lacks IR filters, you will be able to see the emitters glowing purple when then are on.
If you want to troubleshoot the problem even further, please see this post I made in a similar thread:
It has some useful diagrams along with a list of things you can check to try to narrow down the source of the problem.