JBL's are interesting. They're actually warmer and smoother in response than most "guitar" speakers that often have sharp resonant peaks. People refer to JBL's as bright, but really that just comes from the extended treble response from the metal dustcap. That metallic treble is very much a part of Jerry's clarity and sweet top end, but overall, the tone of the JBL as a whole is very smooth. Typical "guitar" speakers can be very sharp and peaky in the mid-treble range. With overdrive and dirt and tube compression, that kind of speaker does really well to clearly deliver and bring out the distortion harmonics, but playing dead clean into that kind of speaker can be brutal and ice-picky.
Think of SRV's "Lenny," a very beautiful and sweet tone, warm, clear, juicy, sparkly, and not the least bit harsh. That was a JBL 15", basically the same voice coil and treble as the 12" like Jerry used, a very similar speaker. People think of JBL's as bright because at low volume the metal dustcap zing is very present and up front. But when you get louder and push the speaker, the paper begins to dominate and show off that smooth overall character and warm JBL voicing, and the metal dustcap is there to keep things present on top and not get buried in a mix.
JBL's can be ugly sounding if you use the wrong kind of distortion pedal. Some pedals have high treble distortion content that really sets off the JBL metal dustcap in the wrong kind of way, buzzy and kazoo like, not pretty.
... and it's just like any other day that's ever been...