KM54 - worth it from a long term perspective?

Yes, I believe there are two investors, holding at least one thousand of them if combined. But I also believe that those are all that is left basically (herhaps a few hundred in total other than their stock), and while it may take a couple decades or more for them to sell off a large part of it, it will happen eventually. And if they manage to sell of their stock so slowly that most of us will be old men when these tubes run out, is it still worth paying perhaps several thousand dollars for a tube that you may have to replace immediately because it was subpar? I mean, chances are that many of the best tubes have been singled out already from when the tubes were cheap and may have already been used in microphones, and that a large part of what is left are the tubes that no one wanted to use when they were readily available. Or am I too pessimistic?

Sometimes you got to roll the dice if you want the rewards. You might lose, but you lost trying. At least that's my perspective. There's magic in the 701's. Moreso than most other tubes IMO.

My 53's have 83 capsules due to the capsules failing in the past before I got them. They are WAY different than 83's. It's the tube.  As rowmat said - hyper detailed.  Nothing else like em.

Tim, and/or Christian bowie, What's a good reliable tube tester for AC701K tubes?

I've always wondered how the nickel capsule Gefells (M295?) compare.

Testers are good for several things, but they won't tell you if a tube is acceptable to use in a mic. For an AC701, you want to do your best to find true NOS/unused, and then run them in a mic (or rig made to simulate a mic circuit) for a few days to see how they behave. I choose not to stock AC701s because of the number of fakes and used being sold as new these days. I install them for customers who already have spares, but I don't actively shop them.

 Though I love tubes and spend many hours every week listening and comparing, I'm personally not into the mystique the community attaches to certain tube types, nor do I think mics have to have specific tubes to sound amazing (certain exceptions exist). Anyone who has contacted me about tubes like this has heard me preach against fetishizing the originality of old gear. I do feel that modern production tubes are mediocre, but had things gone differently we might be paying hundreds for a different tube. The EF800 was nearly forgotten about for decades. A few mic designers started making brilliant mics with them recently and now they're starting to get hot. For mics where a specific tube influences the value tremendously, yes, get an AC701. But, there are a few SDCs where one must decide if it's worth converting and I think people should be less afraid of that. There's an unfortunate mentality in pro-audio that sounding slightly different than an original = sounding worse.  

 And, regarding nickel diaphragms, yes, they are fragile and that's something to consider with these mics.   

Preach!

Then I'm glad I've never been recorded by these as what I am thinking is usually grounds for defenestration.