I have been listening to sierra 2 for over a month now and they are amazing. Superb imaging, clarity, soundstage. Believe me that RAAL ribbons are seriously great. Not a bit harsh at all even at louder volumes. Once you hear those, you will not be able to tolerate regular tweeters. Sierra 2 also has unbelievably good bass in such a small form factor. couple of times i thought my sub was ON. I listen to all kind of music from classic rock, hard rock, death metal, jazz etc and sierra 2 have been amazing for any music i have thrown at them.
clarity and instrument separation is just too good.
I am very sure i will not need any speaker upgrades in future unless Dave comes up with something new .