You have to consider that ANYTHING is only worth what someone is willing to pay. I have a houseful of antique furniture, classic guns, and a ton of vintage audio gear that are "supposedly" worth far more than I paid for them--but only if I find the right buyer. I know what the value of my stocks, bonds, CDs and annuity funds are worth, and I can cash them in at any time and be guaranteed to get that amount of money--right now. If I need to "cash out" my furniture, guns, or audio gear, there is no guarantee as to what amount of money I can/will get, and how long it will take to make the sale at an "acceptable" price. So although these could be considered "investments" in that they are physical assets that have some monetary value, there is no guarantee as to what that value is if/when I choose/need to liquidate them.
The audio market is particularly "fickle" in that certain brands/types of gear come and go in and out of favor (and subsequently value). I agree with many others that the "bubble" is about to burst. All of us that have hoarded and restored all of this gear (and it is a finite supply) will eventually be "downsizing" for any number of reasons, so the "holy grail" pieces (of which you see one overpriced listing on Eprey), will eventually turn into 20 listings of that piece competing for a sale--happens with everything--classic cars, homes, and yes, audio gear.
So I wouldn't bet the ranch or my retirement on audio gear.