This is a complex question and I'll try to answer it the best I can from my experiences. I'll start by saying I own both a skm184 and skm140 set. I too, got the 184's and made the "logical progression" to a more complex setup once I knew why I needed it. I too, loved my 184's and literally bought the same mics again when I got 140's, knowingly.
Page is correct that the km184 and the km140 are the same microphone, the differences are the ability to switch capsules(ak50,ak20,etc,etc ) on the km100 bodies, the ability to run "actives"[they are actually passives but that's another story entirely] by running with the LC3 cables in between the body and the capsule for a smaller "in-air" footprint as well as a -10dB attenuator on the km100 body. You are in luck for a comparison of the 184's and the 140's as taperjoe and I set out to try to do a comparison of the two in a live environment a while back by running the exact same rigs other than the microphones which were my matched pairs of km140's and km184's. Here are the two sources:
http://bt.etree.org/details.php?id=532775http://bt.etree.org/details.php?id=532788pictures of the rigs:
http://205.234.187.219/tab2010-02-19pic1.JPGhttp://205.234.187.219/tab2010-02-19pic2.JPGJoe and I both agreed that the second set was the better comparison due to a beach ball impact in the first couple songs of set 1 that repositioned the mics a bit. We reset them for set 2 and got a clean run. You can draw your own conclusions from the comparison, I don't want to seed your listening with thoughts of what our findings were.
On to the question of running in close quarters, I have run on stage with my skm140 quite a few times now and it is extremely compact. The musicians really never even notice it's there if they weren't there when I set it up. The 184's footprint is much larger, obviously. I haven't found musicians to be intimidated by the on-stage size of it but in tight quarters, it's just not as practical. The 140's run as "actives" with the LC3 cables are the way to go.
On to modularity, interchangeability of the capsules is a very nice feature, hypers, cards, omni's, figure 8, etc, in addition to the ability to run actives. Enough said on that really. Good stuff all around.
Now, as far as the matching of them, yes, it is significant. By being a matched pair the tolerance is within 2dB of the other microphone in all frequencies. This means that an unmatched pair
could vary by more than that and your ear is more likely to notice the difference. I have not run unmatched pairs, I'm sure some others on the boards here have and can give you good testimonials on how close unmatched pairs really are. As to whether they can be matched at a later date, I wouldn't think so, but I've never asked, so that's just an opinion.
John Willet brings up an interesting point with the KM-A / KM-D series, but I would disagree with his assessment that you shouldn't buy km100 series since they're as he said, "old." Being old and still in service is a compliment to any microphone series, tried and true. These have been proven in all field situations for years and years by countless hobbyists and professionals. That's not saying you shouldn't look into the options he mentioned, he makes valid points about the ability to move to digital recording easily with the newer series.
A quick aside to address the km84i's that have entered the discussion, some of the best mixes Joe and I have made are km84i's+km184's, in my opinion they perfectly compliment each other. The 184's lack a transformer, and the 84i's lack the frequency adjustment and in general sound darker(but warmer) than the km184's. Here's a 4 channel source with 84i's in X/Y and 184's on the stacks to hear how they mix:
http://www.archive.org/details/nma2009-02-07.flac16.4chA quick closing thought that goes along with what sparkey had to say. I would agree that if you are going to end up keeping your 184's and ultimately end up running 4 channels that you might grab a skm150 and get hypers right off so you have cards and hypers, as opposed to 2 sets of cards that are the same. As for his suggestion on the subcards to compliment the hypers eventually, that would be a great rig as well. Good suggestion sparkey, I may have to look into that myself.
Alright, I've said enough... have fun with whatever you end up with!