No loss of sensitivity. In fact, you'd have to pad down the APT80 because it's over 10dB more sensitive than the Alpha8. You can use a standard B.worth 3rd order hi-pass @3.5KHz, but I found the values below works well:C1: 3.3uF
L1: 2.5mH
C2: 5.7uF
4 ohm resistors for both L-pad resistors
The figures give you a 3rd order drop at 3.5KHz, 10dB attenuation, AND compensation for the 3dB roll-off of the APT80 in the top octave. (At least that's what SPICE predicts.) Sounds good too. I've tried it with a 13dB attenuation, but sounded too dark to me.
I have the 0.5mH coil in the path of the woofer, but would like to try a 0.25mH. Oh ya, connect the tweeter in reverse.
Gar.