Here's an old post from Dan Kennedy, the designer of the Great River preamps:
Well, in general, a dynamic type mic, including ribbons is sensitive to the load, because it is after all a generator.
When operated into a light load the higher frequencies will typically be extended. With a heavier load i.e. lower input impedance, the high end will drop. At some point the lower impedance will also interact with the inductance of the mics output (be it a matching transformer or the coil itself) and this will cause the low end to start rolling off as well, giving a honky midrangy tone.
With some condensors, those with output transformers and wimpy amplifiers, like KM-84's, original U87's you'll get fairly similar responses plus that always desirable distortion boost when that poor little FET is driving a low-Z input.
Others, like most Schoeps, about half of the Chinese knock-offs and most of the new Neumanns don't really care. The other half of the knock-offs will behave like the KM-84, but worse because they have truly crappy output transformers.
Most mics are designed for load impedances of 1K to 3K for "flat" response.
What's it mean for you? Try it, use it like an equalizer, be sure you aren't causing the mic to puke with too low an impedance and have fun. You won't cause any damage to the mics, it just may not sound good. It might be louder, but be careful with that, listen to the characteristics, not volume.
