Aha! Did you notice the different spelling of Infinity vs Infiniti? The company with the "y" is an audio company - so yes those are aftermarket speakers. They are also "two-way" meaning there is a dome tweeter in the center of the larger cone. That's what the capacitor is there for, it acts as a high-pass for the tweeter. It's possible the tweeter has failed which would make the sound dull, however it would be odd for both speakers to fail in the same way. So you might need to do a bit more experimenting.

The signal from the head unit should be full-range to those speakers. Not sure what to suggest for further testing... I guess because your speaker leads don't have the stock connectors on them you might want to trace them to be sure they are not wired through some aftermarket processor? Be sure they are the original wires coming straight from the head unit?