To start with, 100% antifreeze is bad. It is not supposed to be run without being mixed with water. It does not do very well at absorbing heat and dissipating it via the radiator.

Second, popping the freeze plug is bad. There are a few reasons for that to happen, but I suspect you really really overheated the engine. I would not be surprised if the block is warped (or cracked).

Thirdly, when you say you hear the pop "when turning over the engine", is this via the starter, right? With all the spark plugs pulled and the throttle wide open, right? The popping could be several things and should not be related to the compression, unless you are hearing the rods rattling around on the journals because you ate a bearing.

Finally, it has been said at least 3 times above, you need to do a leak down test. You are chasing something strange and unless you can pressurize each cylinder and see where compression is going, you are just guessing at this moment.

As an aside, I find it very strange that you did all you did above and you have the same compression numbers? Did you clean out the head bolt holes? Oil or junk in them will prevent you from torquing the bolts correctly. Other than that, if you are sure your head is good, the valves are good and the gasket is good, then the compression is going down into the block somewhere.

Good luck,
Michael