Puzzling leak in my 2nd floor tile shower
For any leak, you have to find the source, and also note that leaks travel along surfaces so sometimes where the water is seen is not directly under the leak.  Often this will involve cutting up walls and causing more water damage.

You need to find out if the leak is coming from a supply line or waste line, and also when the leak occurs.

First thing - when there is no water running and no water in the tub, is there a leak?  If so the leak is before a fixture in a supply pipe.

Now plug the tub and fill it up.  While the water is running (but not draining), is there water leaking?  If so, the leak is at or beyond the shower valve and possibly in the shower itself.  You mention that the leak is pretty big, so if present at this point, stop filling the tub and watch to see if the volume of the leak subsides. If so it could be a leak in one of the shower valve fittings or the diverter stem, or it could be water splashing up and getting behind the tile.  In this case your best bet is to open the wall behind the shower (if possible) and look for leaks.   

If the water is still running with the tub full but off then the leak is in the drain pan.

If not, pull the plug and check again for leaks.  If you see water now, the leak is in a waste pipe. This is the scenario where you can end up cutting lots of drywall to find the source.  Start at the tub and go from there, follow the pipe.

If the drywall/plaster is already water damaged, you may as well rip it out before it grows mold - you are going to have to replace it anyways.