Hello Steve!
If you don't really want it as an effect, but as a real bit-depth change,
you might try sndfile-convert. Besides that, Iknow, that Ecasound - and so
most certainly Ardour - can change the bit depth of incoming audio. Otherwise
the decimator plugin is a very good way to go.
Kindest regards
Julien
--------
Music was my first love and it will be my last (John Miles)
======== FIND MY WEB-PROJECT AT: ========
http://ltsb.sourceforge.net
the Linux TextBased Studio guide
======= AND MY PERSONAL PAGES AT: =======
http://www.juliencoder.de