Looking at the impedance and phase angles above I would try a more powerful amplifier as the Avondale amps may not be able to cope with this speaker. Don't misunderstand me the Avondale amps are fine and I actually had my NAP 135s upgraded to stage 3 for a big improvement however they couldn't really cope with my loudspeakers.I agree with this possibility.
The S5e is pretty flat in the treble and doesn't exhibit any nasty resonances in the upper-mid or lower treble:
Another possibility comes from the fact that the the impedance is quite high at 3kHz and shoots again above 10k:
If your amplifier has a high output impedance it may be reacting to the load and the combined response will peak where your ear is most sensitive.
R