Damping Factor vs speaker sensitivity

I would not think so. Damping factor affects mainly smoothness of frequency response, because it is inversely related to output impedance of the amp. A high DF gives smoother frequency response on a speaker whose impedance varies with frequency (most of them).

DF is frequency dependent and conventionally measured at 1 kHz. Unless you have a graph of DF showing it is 4000 at higher frequencies, it is probably lower there (though maybe not relevant to your question).

I don’t know if this applies to your situation, but sometimes distortion in the high frequencies can masquerade as more extension. If the Hegel has lower distortion than a previous amp, it could give the impression of less extension. I've not listened to any Hegel amps, so I don't have any insight there.

There is inductor resistance, voice coil resistance, resistors, capacitor impedance, inductor impedance. Your real damping factor is never anywhere near 4000.

Damping factor is pretty much meaningless unless one is talking about a tube amp with extremely low damping factor, like around 4 or less.  Beyond 8, the numbers don’t really matter that much unless the speakers have a really low impedance.  

theaudioamp, I have already seen >900 DF on a Hegel publication. Looks like they changed the way they measure it?

mike_in_nc, May be a greater smoothness give the impression that the high frequency did not sparkle as it is suppose to be, for my taste ?