Vertical position of rests in DrumVoice

[David Sumbler]

I am having difficulty controlling the vertical postioning of rests in
a DrumStaff which has several DrumVoices representing different
instruments.

The easy method of using a note-name followed by '\rest' doesn't seem
to work in a DrumStaff.  I have tried, for instance, 'sn8\rest', but
this produces a syntax error.

The other method I tried is overriding Rest.voiced-position and
MultiMeasureRest.voiced-position in the relevant Voice context, but
this seems to work inconsistently - something else is overriding it. 
And in the case of the MultiMeasureRest nothing I do will move a whole-
bar rest below the bottom line of the staff, even though in the same
voice minim rests are being placed one or two lines below the staff by
Lilypond itself.

How do I take full control of the vertical positioning of these rests?

David