Slate1
I own a pair of LSi15s and at first I thought the tweeter was kind of laid back too. Then I removed the grills, gave them so toe-in, and replaced the flat jumpers with some quality ones(Maybe you already tried all this), and bingo, it was perfect. I would also recommend giving them some time to break in. The thing about that tweeter(IMHO) is that is so damn accurate, and uncolored, it kind of takes time to get used to it too. It doesn't call any attention to itself, but at the same time, it gives me all the detail there is to give in a totally unforced manner. Once I figured this out, others speakers seemed bright (maybe even a little hard) by comparison.
Again removing the grills, and replacing the jumpers gave the LSi15s the little bit of sparkle I thought was missing, without making it bright or anything like that.
If that still doesn't work for you, maybe some of the recommendations that were given may be better choices. I would also recommend you check the Amphion Argon2...
Just for reference, if it helps I'm using Analysis Plus Silver Oval cables(which I bought used here on Audiogon for a good price) and Analysis Plus jumpers. I'm also using a Perreaux Radiance R200i integrated amp(Awesome amp by the way) and a Sim Audio Nova LE CD Player. As for the kinds of music I hear(just for reference)...I listen to a lot of different kinds of music Pop, Rock, Jazz(specially like CDs where trumpet is the showcased instrument so to speak), Vocal Jazz, Latin Pop, Latin Jazz, Spanish Rock, R&B, and even some Hip-Hop...I sometimes use the song Ants Marching from Dave Mathews band's "Under the Table and Dreaming" CD to kind of get a handle on how my system is sounding.
Hope this helps...