she thinks I sound better singing high up; I think I sound better low down

When I'm practising songs there always comes a point where the song finds its pitch for my voice, which usually means going up as high as I can manage. I'm interested that you find the timbre of your voice changes a lot, though. I find that I can sing more or less the same notes as mostly chest (Boney's Lamentation) or mostly head (The Unfortunate Lass); which one I use depends on the song. Perhaps the difference is that you've extended your 'head voice' register beyond what your 'chest voice' can manage - in which case I'd love to know how you did it!