It depends on what supertweeter you use. If it's a low-impedance ribbon that could give your amp a pain in the, ah, output stage.I use FE206E's with a T90A on top, padded down somewhat, and rolling off first order below 15 kHz (still contributing significantly an ocatave or so below that point). I don't use a high pass filter on my FE206E, but I do some response shaping down in the lower treble region. The FT17H is also a very nice unit at a much lower price, and if you're going to cross over that high it's probably just about as good.
Duke