dashes, again

My subscription to the list was interrupted, so I am trying this 
query again. My apologies in advance if you are receiving this 
message for the second time.

I'm a graphic designer, and the differences between an em-dash, 
n-dash and hyphen matter to me. I customarily use – for 
en-dashes and — for em-dashes. When I clean up my pages using 
Mac Tidy, though, it converts these both to the same hyphen. Same 
thing if I use –  — I use a configuration file, but 
haven't been able to find what variable controls the way Tidy deals 
with these entities. What is the solution?

Thanks!
Chris von Rosenvinge