Well, if the DF is not sufficiently high, even small loads on the speaker cables or a sufficiently reactive driver can create havoc. This is less of a problem with modern designs (especially digital amps), but excessive resistance on speaker cables can definitely become an issue.

I'm certainly not qualified to wax philosophic about amp designs, as that isn't my field, but I do have a basic understanding of the physics involved and a more than passing level of experience in the music recording industry. Probably just enough knowledge to get me introuble... 8-)

Cheers,