CV,
HDF (high-density fiberboard) has a denser core than MDF, but it isn't necessarily superior to MDF. The reason is that if you use too dense a material and/or over-brace a cabinet, you move the potential resonance to a higher frequency and the "Q" of the resonance gets larger--so it may become audible. (The Q of a resonance is its amplitude or relative loudness.) Note, too, that it's impossible to totally eliminate box resonances. Everything has a resonant frequency and it becomes a matter of controlling them.
The ideal is to create a box or enclosure whose resonant frequency is at a very low frequency and the "Q" is small. Research we've conducted at Axiom has shown that MDF and a moderate amount of bracing is ideal at keeping any box resonances below audibility. If you add too much bracing or use a more acoustically inert material, the resonance frequency moves up into the audible range and increases in amplitude.
So the other manufacturer who uses HDF may have bought into the folk wisdom that a denser material and more bracing makes for fewer resonances, or it may be a marketing ploy.
Regards,
