Rwwear, FWIW they were never one of our dealers.
The Fertin full-range driver is an extremely high quality driver, yet it is not driven well by a 'constant voltage' amplifier like the Krell. The manufacturer specifies 'current source' amplifiers for its use- these are amps with a high output impedance.
Another example is the Acoustic Research AR-1, the very first acoustic suspension speaker made. Acoustic Research specified that the amplifier output impedance should be high for best results (seven ohms).
The simple fact of the matter is not all amplifiers will drive all speakers properly, and it is not a failing in the amplifier that that is the case. It is simply that there is more than one approach to creating audio reproduction.
Note that this is not a conversation about tubes/transistors; as an example of what I mean, look at the Nelson Pass First-Watt amplifiers, which are 'current source' devices, intended to drive speakers like the Fertin mentioned above, which they do very well. So this is a matter of intent by the designer, and the designer will be the first to tell you that there is no amplifier that can drive all speakers.