I'm a bass player myself, and sometime ago I added a Bass Wah (and Bass vocal wah) to the Q-Wah algorithm b/c I found it lacking on the low end.   After doing that, Q-Wah has worked pretty well for me for an envelope follower as well vowel sound wah, either sweeping a single vowel up/down or morphing between two vowels.  Anyway, personally, I prefer Q-Wah to the ModFilter.