I've mic'd bass cabs with the Sennheiser E609 instead of a 57 with really good results, depending on the style of music of course. It goes to 18KHz instead of the 57 which stops at 15k, its probably better for micing a 6". The sound is a little more tame than the 57, though. Its more of a natural sound. The 57 sounds a little more "alive" and colorful to me, But I like the normalized sound, and even-frequency of the E609. At least for clean bass recording. If you were playing distortion or heavy rock. I'd go with a 57, for sure. For a smoother sound, I like the E609
The E609 is also made specifically for recording amplified speakers so its not as picky on placement - It will pick up the whole character of a speaker cone, where as the 57 will get mostly the portion of the speaker its aimed at.
I have always used DI for low end so I cant comment on the RE20 or other nicer mics.
oh yeah, guys, dont forget to take the metal grill off if ur gonna record the cab, it vibrates and responds to different frequencies of your cab. It will sound fine 99% of the time but that one specific frequency can resonate the grill and ruin the one important part of the recording.