The 'classic' distortion sound is produced by overdriving the power stage of a tube amp. Its a great sound, but it doesn't change much, unless you have some kind of mic>effects>PA setup. Preamp distortion, or stompboxes, allow greater flexibility with things like EQ.

I gave a DS-1 to a bass player I work with because he needed some signal boost to overdrive his amp. I can get the same type of sound out of my Peavey Rock Master preamp, with more versatilty. The Rock Master is a first-class piece of kit, but its out of production, so its getting hard to find.

I've gotten good Bluesy tones with a Boss OD-3 overdrive pedal, and I used to use an Ibanez SM-9 Super Metal pedal for that 'doom-bringer' sound. I've heard enough good stuff about the Tube Screamers that I feel they're worth a look too.