the u87's hpf filter starts up pretty high. so i don't think the 100hz hpf on your avalon would do much. curious what distance your singer is to the mic? since the u87's hpf is pretty strong, i think it works best when you're right up on it. it seems to tame down a strong proximity effect and make that sound more balanced. however, when micing more distant sources, which don't have proximity effect, i think the hpf makes such sources sound too thin.