Ben, the KM 140 (and dnuggs, the KM 184, which is acoustically identical but electronically a little better, although not modular) uses the "innards" of the same capsule as the KM 84 did, but the shape and dimensions of the rear sound inlet are different. This causes a slight, further elevation of the high frequency response, only around 1 dB, but it's a narrow elevation (a/k/a a "peak"), which for me was a problem when I briefly owned a pair of 184s. The only analogy I can make is to the taste of saccharine or aspartame, which I find really annoying, although a lot of people think that they're just fine.
--best regards
[Edited later to add:] That last sentence was a little dismissive in a way that I don't think Neumann ever deserves. Let me try to put things in perspective.
- If the KM 84 had never existed, and the KM 184 (or something that sounded the same) had been Neumann's small cardioid offering in 1972-73, it would still have been a real step up from what I'd been using before (AKG C 451--the original version, not the "tribute" electret that they came out with in recent years; the old series had a very nice omni capsule but a rather hard-sounding cardioid). Professional singers already have a lot going on in the high frequencies, and any further boost in the microphone--at least in the kind of recording that I do--is really off-putting. It might have made sense back when the "delivery media" (phonograph records, pre-recorded cassettes, AM radio) had audible high-frequency limitations, but certainly not nowadays.
- The directional pattern of cardioid microphones is never exactly the same at all frequencies, so what is picked up from the room has a distinctly different coloration than what's picked up on axis. Typically the frequency response from the hemisphere behind the diaphragm is "bathtub-shaped"--it rises both at low and high frequencies, then falls off again at the extremes. Cardioids are really the worst in this respect of any first-order microphone pattern. That's why I generally avoid them in favor of in-between patterns such as "wide cardioid" (or Schoeps' "open cardioid") and supercardioid. I've also learned that if cardioids must be used, side-addressed cardioids can be audibly better in this respect than "end-addressed" cardioids.
However, as I said about the KM 140 / 184, many people really like the added brightness/brilliance. I can very well imagine that if I let myself get used to it, any microphone that didn't have it would seem to be lacking something.