Well... Several years ago, I had a broken TFS valve spring. The car did lose power, but also I could tell that something was not right at all. The sound and engine vibration gave it away. Hope this helps some.
Well, this might sound crazy, especially with the amount of technology avail... but...
Several years ago, I cut the wires to the check engine indicator rather than chase ghosts and gremlins. I work on my stang by sound, feel, and smell. I have a good idea when "she" isn't running right and I can usually tell pretty quick if it is spark, fuel, or for some other reason. And for this particular case, the engine vibrated differently and the sound of the engine sounded different, I can't explain what it sounded like, but I knew it didn't sound like normal. I know this isn't what you had hoped for, and it has been enough years that I don't remember enough of the sound to describe it. Sorry.