To think of audio gear in terms of appreciation/investment is not as straightforward as, "How much did I pay and how much can I sell it for?" First, you have to take into account the initial purchase price, what you could have done with that money instead, and finally, inflation. Not to mention the cost of gear upkeep, and current cosmetic condition, which is an important factor when selling anything.
Let's look at a fairly desirable unit from the 'silver age'. Nothing special, but still fairly interesting. Something hi-fi nerds might not mind owning. A Pioneer SA-9500 integrated amplifier from 1977. I'm familiar with that unit since I've owned a couple. In 1977 I bought new, mail order, for about $450.00 (I think that price included the walnut sleeve).
According to Inflation Calculator, that is $2,225.00 in today's funny money.
Looking at the on-line marketplace, the highest asking price for an SA-9500 is $2000.00. Most are around the one thousand to fifteen hundred dollar range. This particular unit is said to have been completely overhauled. New capacitors, transistors, protection circuitry rebuilt, and so on. Cosmetically it looks pretty good. Not new, but better than you might expect. It is 'naked' but someone is selling a walnut sleeve for several hundred dollars.
So, has this piece appreciated? No. Not really. You could say it has at best maintained its initial dollar cost. You could also say that over the years it has provided the owner with good service, so that is something. You might even argue that for the price, it is in many ways more desirable that what you can buy, new, today. YMMV in that aspect.
Apart from some unusual 'historical' gear--Saul Marantz' personal amp, certain Mac items, and maybe (but questionable) something like an LNP-2 or Cello Palate, there's not much hi-fi out there that is selling for more than it was new, all things considered.
It would be interesting, perhaps, if someone had a list of items sold for more than they were originally offered, taking inflation into account.