“i.e. to improve tone, do you need to split the resonance between the mouth and nasal cavities? (I know there are other resonators, but just for the ease of the question). If so, how do you achieve this? Is it though manipulation of the soft palette?”
The Relaxed soft palate is affected by posture, head positioning, outer throat tension, inner throat tension, jaw positioning, volitional effort, and more. The sound travel path is also affected by head positioning, relative to the throat and its angling.
Without getting these many factors right, it is challenging to hit the right vocal spot that enables for head resonance (which is primarily the mask area– the sinus cavities and some of the nasal cavity). And without the Relaxed soft palate, it is very difficult to keep this vocal spot ringing.
To get the soft palate right, begin with fixing what affect it– the throat underneath and the head angling above. This is done by posture changes, and is a difficult process, as I will later describe in http://www.vocalposture.com.
Regarding nasality, killerku’s videos explained well that nasality works for many kinds of songs. In general though, until one has developed a commercial style, aim as a goal for a full sound. This means, using both mask and bass resonance together.
The difference between nasality and mask singing is that one actually sends a bit too much air through the nose and uses less of the sinus cavities for resonance. This is less than optimal, because: 1. it is not the goal of the full sound 2. if send too much air through the nose, too much warmed, moist, slightly acidic air go through the nose and this isn’t healthy 3. the sinus resonance developed is too sharp of a sound 4. mask singing is a crisper higher that accompanies a mouth sound far better.
So, to begin with, aim for full sound with sinuses resonance, by first straightening out your posture, in a relaxed way.