Had my first up close experience with the dust cap on my JBL K120 yesterday. Sound check was fine. Started playing 1st set and was getting just an awful high-end brittle tone from the rig. Thought maybe something had changed on the board to mess things up. When I looked down at the speaker cab I saw that the mic had been knocked out of place - it was sitting dead center over the dust cap. Had always heard the dust cap was very trebbly and brittle - but man - this was so bad it was hurting my ears. Moved it back to the edge of the speaker and back came a much nicer tone.
Amazing what a huge difference this made. Night and day. Just goes to show once again how every piece of a setup makes a difference. It's not just what you buy, or how you play, but where you put it too.