I guess I know what Snejk means. I also had a time, that I was concentrated very much on high notes that my lows get's a little lost. It's not about vocal fold's and warm up, but obout vocal tract setting and muscle memory of it. Today I'm playing with many different settings of vocal tract -extremly bright or dark,fry and whistle and effects- creak, distortion growl and mixing of it. And despite I do not practice range itself - I do find it get's pretty wide :)