Guido - I don't know the Whisper impedance curve, but they are rated at 4 ohm nominal and about 95 or 96 dB efficient, I don't remember off the top of my head.
The Stereophile review seemed to start well, but then MF began some deep criticisms mostly in comparison to his tube amps as I remember (but my memory doesnt always serve me well). I was informed that the amps he had were brand new and unbroken-in, but you never really know. Even my amps sounded sterile and flat when they were new which matched his observations. I left them playing when I went on a trip to get the hours up. Now they are really nice. I wouldn't have believed this burn in thing if I hadn't noticed it myself.
In the end it always seems to be a question of synergy anyway, as much as that complicates matters and frustrates us.