I was trying to fix a broken 40" Matthews c-stand (spring-loaded model, legs were frozen up) and I, too, had a problem getting the lock nut off the bottom, so I searched for an answer and came up with this thread.
Although all of these ideas are decent and probably work just fine, I have a stupid simple solution for removing the lock nut:
1. Remove the telescoping sections from the main tube.
2. Get a 3/4" (I think this is the standard nut size for Matthews c-stands) deepish socket and an impact driver (I used a basic 18V Makita, nothing fancy).
3. Pour a few handfuls of copper BBs (or lead shot or maybe small sinkers) into the main tube.
4. Keep the stand vertical and remove the lock nut with the impact driver.
5. Try not to get excited and dump your BBs all over the floor when the nut spins right off.
6. Tilt the stand and pour the BBs into the container they came in (I had a carton) or whatever.
Just reverse these steps to put it back together. And when the nut is back on, you might have to bang on the tube with a mallet to shake the BBs out... otherwise you'll have a very annoying c-stand maraca. ;)
