Unless it was a targeted theft, I cannot see anyone lifting a 300 lb box unless they know the value. I have my own, and dread moving them! And logically, who would break into a garage for scrap? Any copper is in the walls, and not an ideal/easy item to grab.

I agree with many others- you know the thief, or friend of the thief-

Either way, I hope you get it back. Or worse case get compensated for its real value.

No reply or update from the OP, so I will add my two cents about the tangent topics-

Over the last 15 years, I have had 3 major insurance losses in my life-
1) Moving company truck (18-wheeler type) burned to ground when I moved.
2) House took direct hit by lightning, resulting in losing all electronics, HVAC, computers, etc.
3) Hurricane/Hail damage to house, which required roof replacement

Perhaps I am in the minority, but all were resolved successfully, no insurance hassles, and "Replacement Value" basically replaced old with new. It took a lot of work, patience, and documentation, but at the end of the day I was more than satisfied. So I will disagree that all insurance companies and their agents are terrible.

Biggest lessons I learned, especially after #1:
1) Document all purchases. I charge EVERYTHING I can now, so I have a formal record of everything purchased, and when it was purchased. Credit card annual spending reports are also backed up offsite. I also photograph stuff periodically (mostly before hurricanes).
2) Periodically reevaluate your policy.

On a separate note, is it really a surprise that an insurance company will not scratch a $20,000 check for a pinball machine, when they didn't even know that the OP owned one? See #2.

Looking forward to an update from OP.