They are easily modified for bass. Change some input/output caps, clipping diodes, and a resistor for more (or less) gain and you're all set. Keep the tone control in the first 1/3 of the knob to keep mids so you won't get lost. It works even better as a boost as it has a lot of volume- you can roll off the dist and crank the level and really hit a tube amp hard. I've been using one for years.
Everything you need to know about modding them, including schematics, can be found @ diystompboxes.com in the forum (do a search)
For a $40 pedal with some mods it's a great addition.
Modifying the DS-1 for better bass was covered here:
Boss DS-1 modded for bass CLIP included.