http://www.theaudiobeat.com/equipment/vtl_mb450_iii_mb185_iii.htm
What’s the significance of adjustable damping factor? Far greater than you might imagine. The clue lies in the fact that it is derived from the degree of feedback, the higher the feedback level, the greater the damping factor. In this day and age it is easy to assume that all feedback is bad and that the less you have the better, but it isn’t actually quite that simple. There’s a world of difference between local or nested feedback (used within a circuit) and global feedback (that encloses it entirely). While feedback-free preamps are all the rage, feedback is critical to a power amplifier’s damping factor, just as Q is critical to a speaker’s voicing. Between the two, these factors have a huge influence over amp and speaker matching, and as Q is generally a fixed value (unless you happen to be using the large Focal Grande Utopias, with their adjustable-Q EM drivers) then being able to adjust damping factor to achieve superior matching is a major benefit, significantly broadening the type and number of speakers with which the amplifier can achieve really good results.
VTL offer four settings for damping factor: Low, Medium, High and Max. With any given speaker you will find that one setting is dramatically more musical, dynamic, involving and better integrated than the others. In my room and in most cases, I settle on either the Low or Medium setting. Those who crave a drier, more heavily damped sound might opt for the high setting, especially if the speakers are a little loose in the bass or the room is exaggerating the bottom end. I can’t really see anybody needing the Max setting, but it serves a useful purpose in establishing the continuum along which you are operating. Once you’ve heard what Max does to the music, you’ll treat the setting of the amplifiers’ damping factor with the appropriate caution and seriousness. It not only has a huge impact on performance, it offers a huge advantage in terms of choice and your amplifier’s longevity, should you (or should that be, when you) change your speakers at some point in the future. If the amp/speaker interface is the single most critical junction in your system, the Series III Signatures pull the neat trick of offering genuine adaptability, a serious benefit that’s not to be overlooked.