Exactly..."the way the strings are attacked over time to give the guitar a kind of expressive quality". I for sure get that and with all the amps I have, that is what needs to happen in order to get there...heavy right hand, to produce that effect. Some more than others.
But, some amps seem to inherently possess more of that vocal/vowel/cocked wah/kerang thing in the voicing, to me anyway, and the heavy picking attack just takes it to the next level. Some amps sound much flatter, more neutral regardless of how hard you pick. I guess yet another term that I relate to that kerang/cocked wah adjectives in a sense is growl...like when you smash a power chord. But it also seems to be a combo of the two things mentioned above...the way the strings are attacked and the inherent voicing of an amp. Two examples using Wizards in my experience: MCII has kerang/growl/vowel like tonal quality, especially when you really hit hard with the picking hand. The MTL I owned did not and came across very neutral in that respect regardless of how how hard I picked, and thus bland for my taste.