Other hobbies with these economics?

Guns keep their value, especially sought after ones. Custom knives and custom pool cues also appreciate in value as well. And then there are comic books. I bought some #1 issues of Vampirella and Creepy last year. Boy I wish I still had that Spider-Man #1 I sold in 1979. I think that thing is worth about $16,000 now. Oh well.

beer steins. audiophiles always know what they have and buy and sell according to current market value. we are all happy with our sales, purchases and goosebumps. stein owners typically don't appreciate what they have because somebody somewhere gave them this thing that just requires space to store, hence they can be purchased for pennies on the dollar at the local rent-a-space antique mall, brought home and enjoyed while they appreciate. it is too bad that the black plague is responsible for the existence of the beer stein but it sure is nice to drink a polaner out of a 1k stein that you just paid 70 bucks for while sitting in front of the hi-fi.
peace.

Well, if you liken our hobby to others of similar enjoyment, then it would be those hobbies that feature "aging with grace", beauty, adjustability (tweaking), functionality, and oftentimes investment.

Those who collect postage stamps, matchbook covers, Coca-Cola bottles, etc. need not apply.

But many collect vintage motor-cycles, arms of many varieties: e.g cross-bows, alcohol dispensers: including the beer steins above and beer taps (I just missed out on a HALES tap to match my "Hales Heavy" placard...over my T8's), table-top radios (I have 30 odd from Crosbys to Lil' Pups), cast-iron farm animal lawn sprinklers from the early 1900's (these can fetch thousands!) pool tables, musical instruments of course and on and on.
I like to pick-up the odd item that will accompany my audio stuff...like 1/18 scale CART and F1 cars (Nuvolari, Clark, and Ma. Andretti cars in particular).

What other unique collectibles do Audiogoners love to keep?

I have two Japanese Samuari dolls dating back to 1956 and 1965. Both are all Porceilin with cases. I had a chance to buy a female doll made in 1912. I turned my back for a split second and it vanished.

I agree with firearms as noted above. And especially if fired no more than moderately and well cared for. Some collectors just buy to collect and NEVER fire them, but I personally can't see doing that-- I want to know more about the firearm. Good firearms actually hold their valve much better than good stereo equipment. But I understand your point about "daily use"-- stereo equipment would excel there. Cheers. Craig