I'm with HiRoller
on this; there's probably some frequency masking going on. Barry White has a deep (but not dull) voice that comes across fine in recordings. The other instruments have to leave enough sonic space for the vocals, no matter what register they're in.
OTOH, I'm with where
It could be that dull & deep just won't cut it on this cut. Improving dull to vibrant will probably help, but if there's frequency masking it's going to be hard to work with deep no matter what. Any objections to using a different singer that is more vibrant and less deep?