The Cornwall is such an excellent speaker at what it does. For a rocker on a budget it is first class. You do not have to worry about your amp, just about anything will drive the Cornwalls to crazy levels. Down the line if you cross to subwoofers between 80 and 100 Hz you will really be cooking. I have never listened to the JBLs so I can't comment.
As far as forgiving goes I'm not sure I can give you a meaningful answer. Horns are very detailed if you feed them trash like a bad pressing played with a less than stellar cartridge you are not going to like the sound. If it is just a poor recording then you will be fine. The Cornwalls will not mask anything. It is important that your program sources are up to the job but that is true of any fine loudspeaker.
As far as forgiving goes I'm not sure I can give you a meaningful answer. Horns are very detailed if you feed them trash like a bad pressing played with a less than stellar cartridge you are not going to like the sound. If it is just a poor recording then you will be fine. The Cornwalls will not mask anything. It is important that your program sources are up to the job but that is true of any fine loudspeaker.