After some time spent A/B-ing both the Copper's and Tributes, I've come to the following conclusion:
Treble: The Tributes are definitely a tad brighter than the Copper's, and thus are ever so slightly more prone to sibilance. Sibilance on the Copper's is next to nonexistent, though it's rarely ever an issue with the Tributes. I personally prefer the Tribute's slightly more prominent and pronounced highs, as it's not at all excessively bright and only adds life to the track.
Woah woah woah. I think you have the Coppers inserted too deep. They like more air to breath so try pull them out further on the verge of falling out. you'll find your treble there. A bit too much treble if you ask me.