(the plugin takes the frequency you set and accentuates the octaves of that frequency across the entire low end, well into the 300 hz area great for bass guitar) parameter to 120 or above, which brings out the low mid frequencies. Recently i've been setting it to 85 Hz and the kick just sounds oh so beautiful. It produced an unpleasant, artificial sound that seemed incongruent with the kick. I would set the frequency to 50hz, supposedly where the 'feel' or 'movement' of the kickdrum is. When i first got the plugin, i got it because my kickdrum always seemed to lack punch despite eq'ing, compression, transient designers, etc
I've found what works with me, especially with kicks, is setting the frequency parameter to something above 80hz, usually. It works great, say, to have a bass line shine on an iphone's mono speaker, or a macbook's speaker system, both of which have shitty bass representation.
The point of the plugin is to use harmonics in octaves / intervals of 5ths to accentuate low bass frequencies and give them more presence in the mix without actually boosting the bass frequencies themselves. I thought i'd share my experiences with MaxxBass, Waves' bass harmonic enhancement plugin, because it truly is wonderful.
