=== 10kHz Filter ===

Series Capacitance = 1.0uF
Parallel Resistance = 16 ohms

freq resp
=============
20kHz -1 dB
10kHz -3 dB
5kHz -7 dB
2.5kHz -13dB

That will give you some subtle "sparkle" and is probably what you're looking for.

Alternately, I'll describe the exact filter you've asked about, which is a first order 20kHz network. If this is what you want, you'll need a capacitor that's exactly half as big as the one listed above. So you'll want a 16 ohm resistor across the tweeter, and a 0.47uF capacitor in series with the tweeter/resistor connection.

This will give the following response curve:

=== 20kHz Filter ===

Series Capacitance = 0.47uF
Parallel Resistance = 16 ohms

freq resp
=============
20kHz -3 dB
10kHz -7 dB
5kHz -13dB
2.5kHz -19dB

A KSN 1038 using this second filter might add some "air," but it will be crossing over pretty high.