I've actually been having a good time with Ignite Amps' The Anvil plugin for this task. Do the old three-bass-tracks, Pick / Mids / Lows, distortion on the mids... gets a nice grind.
Guitar amps usually create the best/most usable bass distortion in the real world, so if you have any around you can always try to run your bass tracks through them. Just make sure to turn down the 'bass' knob on the amp a lot!