If you have a very good tweeter, a pure Class A wide bandwidth amp of 5 to 20 watts would be nice.
Pass had a project one years ago, that I made which drove the treble panel of Quad ESL57's and a Kelly Decca ribbon as also >12khz
http://www.firstwatt.com/pdf/art_classa_20.pdf
He may have done a First Watt one since, these are the perfect tweeter amp, even mid range as well, in a 3 way as you have.
BTW the First Watt B4 is a killer active Xover
Cheers George