As the VTL literature explains, achieving higher damping factor/lower output impedance, if done using negative feedback (as is often the case), can come at the cost of reduced sound quality above bass frequencies. A good reason to either bi-amp, or use a sub with a dedicated amp and a separate amp uncompromised by damping factor considerations for the main speakers. That, or chose a speaker not requiring a low output impedance amp to sound as good as it can at both low and higher frequencies.
