No clue why he did that, that's a short/fire waiting to happen. There is a reason that ground prong is there, and its not for stability. If your system got better after that prong was removed then you may have some electrical issues with that wall/socket.
That socket tester was the best buy out of all my tools, I will sit at the warehouse on load out day and see people bring in 15-20 supposed Dead Receivers only to test them and they are fine. A lot of service calls I have gone on where 2-3 techs have been out there.. replacing cables, boxes, connectors etc... are just a faulty outlet. The receivers are more prone to reacting to small surges moreso than a radio or dvd player is and even though you may have a hot ground your surround may function just fine, but the receiver will shut off on you, go sfss and eventually burn out. When you replace a receiver and the outlet is bad, it will work great again for a few days, simply because you have fresh electrics to burn up.
If you want some interesting reading about why that 3rd prong is there ad why some systems (newer ones) only use 2 prongs, read this
ELECTRICAL GROUND: Why Three Prongs?