> neumann says the 184s are quieter, less distortion. which some transcribe as "brighter" as that distortion is typically in the lower registers. all that noise is what makes the km84 sound soo warm and smooth.
Sorry, no. Neumann says the 184s are quieter than the 140s because, well, they are--by some 3 dB, for the ones produced since 2002. But they've never claimed that the 184s have lower distortion than the 140s, because, well, they don't.
What makes the KM 84 sound warm and smooth compared to a KM 140 or a KM 184 is that again, it really is smoother in the entirely objective, measurable sense of its frequency response being less bumpy up top. Neumann consciously moved in a certain direction in the design of the KM 140. That model has the same capsule as the KM 84 as far as the membrane and internal parts go, but it is set in a capsule head arrangement that includes a slightly differently-dimensioned rear sound inlet. This causes an increase of about 1 - 2 dB in sensitivity in the region around 9 kHz. That tweak gives these microphones an attractive "sparkle" under the right circumstances--but there are also circumstances in which the emphasis can be undesirable.
Now that, I have to say, isn't Neumann's fault. They didn't make the laws of the universe--one of which is that no possible tweak or gadget can ever make everything sound better. If that were possible, whatever tweak it is (in this case, a little EQ boost that's achieved acoustically rather than electrically) would, by definition, continue to make even its own results sound better and better and better as you continued to apply it a second, a third, and a 473rd time to the same recording. Since that is obviously absurd, so is the idea that any "feelgood box" or anything other than flat frequency response, low distortion and low noise can ever be inherently euphonic all the time.
What Neumann quite reasonably did--because those dear people had this crazy idea that they wanted their company to exist for a few more years if possible--is gamble on how best to succeed in an audio marketplace which is, how shall I say this diplomatically, largely driven by people who have more disposable income than engineering experience. If you listen to a "comp" and you aren't comparing the recording to the live, original sound, a KM 140 compared with a KM 84 will seem to have higher "resolution" and its sound will definitely "cut through a mix" better and so forth. That's a lot of what sells microphones nowadays, and Neumann decided not to turn their backs on the opportunity.
OK, but finally now to face the original question, though, prepare for the anticlimax: There is no difference in sound between the KM 185 and the KM 150 as a series. Sorry, it's really that simple; no contorted explanations are in order. The KM 185 is 3 dB quieter but you will rarely be fortunate enough to hear that difference because the noise of most recording venues covers it up.
--best regards