it's not cheap or that easy to get hold of, but the wounded paw [url="http://www.woundedpaw.com/catalog/product_info.php?cPath=27&products_id=150"]battering ram[/url] is by far the best overdrive/fuzz pedal i've ever used.
NO loss of bass, lovely overdrive to distortion and great fuzz too.
the main downside is the cost and ordering one from canada.
the upside is its ability to keep the low end intact, something i found amiss in most pedals, even those like the fulltone bass driver.
EDIT: don't forget bassfuzz.com for a great round-up of the fuzzy end of bass.