David's question about butterworth filters evoked some
old-time memories from the time (some 20 years ago) when I was 
designing analog synthesizers and other electronic music 
equipment.

One thing I remember from that time is that cascading lower
order filters does not result in a higher order filter *unless* 
you include some form of feedback.  The effect of cascading, for
example, three low-pass second-order Butterworth filters is
a low-pass filter with a slope of 36db/octave, but with a very 
round cut-off region (i.e. the steep slope doesn't start right 
away after the cut-off frequency).  The feedback helps to make
this point "sharper" (This though to explain with pictures and I 
don't want to bore you with 10k words ;-)

I used to be able to design filters and work with poles and zeros.
It is part of standard analog filter theory, so may be you can find 
a book about it.  If I can find my notes, I might want to try it
out myself, which I will let you know.