...do great speakers increase in value over time in the vintage market?

if you keep. Them for 20 years  the value may be more then you paid for them 20 years back

Yes and no . Depends on if the speaker was any good to start with . Many of these keep their prices close to what they cost when new and some even surpase it . Some of these are :
JBL models
Quads
DCM time windows
SPICA TC50
PROAC models
SPENDOR model
ROGERS model
AR models
DYNACO A25xl and many more .

I think old speakers appeal to feelings of nostalgia — remembering good times in your life when you were rocking out and partying with friends or a special girl to speakers like these.  It’s the same with vintage cars (you have to imagine if you weren’t even alive when they were manufactured).  But, I’m not sure how useful those old cars are if you mostly keep them garaged and covered except for a few times of year when you take them to a car show.  At least with the speakers, you aren’t afraid to use them!  And you don’t necessarily have to choose between old equipment and new, judging from the people here who have multiple sound systems.  Enjoy both!

The best example I can tell you of are GOODMANS OF ENGLAND - 
"AXIOM 80" Loudspeaker driver. We are talking mid 50's to very early 60's. Every senior loudspeaker engineer and audio entrepreneur from the early days of high quality audio equipment design and manufacturing, that I have ever had the pleasure of making their acquaintance, agrees on one subject when it comes to loudspeaker design.  The AXIOM 80 10" full range speaker was the most unique design ever invented. I could go on for pages describing the incredible sound produced with this driver. Unfortunately in those days, there we only a handful of companies worldwide that designed and manufactured complete loudspeakers packaged in carefully designed cabinets. Goodmans never attempted to go that far in the US market, only selling the drivers themselves.
The first time I heard one was around 1955 at MUSICRAFT on the north side of Chicago. One of my CSO associates took me up for a listen.
What I heard, considering the associated source electronics available in those days, came as close as I have ever heard to reproducing a violinist as if he were standing in the corner playing  live.
In 1956 at the Chicago Hi-FI show, I heard an AMPEX demonstration of 3 track tape. The loudspeakers they were using were 3 custom made enclosures, each mounting 4 axiom 80's, for each of the 3 channels.
To this day, I have never heard anything at any price that came as close to live performance. And, I have heard and owned a lot of equipment over the years
The following week I went out to MUSICRAFT and bought (8) eight AXIOM 80 drivers with the intention of building 2 of the enclosures heard at the AMPEX demo. The cabinet plans I acquired from GOODMANS of England. Unfortunately as circumstances would have it, the project never happened. A few years later I sold the drivers for $200.00 ea. Oh, and by the way, I still have the original purchase invoice. I paid $65.00 ea.
Now, to answer your question, I have rarely seen an axiom 80 driver on the market in recent times, that wasn't used and abused beyond usefulness. I have seen, on two occasions, (2) and also (4) AXIOM 80'S guaranteed to be in pristine condition. The photo's qualified the claims.  In one case, the asking price guaranteed and tested, was $3500.00 each. In the case of the (4) drivers, claimed to be mint, stored and never mounted. (Beautiful high quality photos), the asking price was $7500.00 each. This is just one example. I hope it answers and exemplifies your question.

High-Quality items have a better chance of retaining value. Speakers although a bit hit and miss still sometimes retain great resale. Tannoy, especially the 15in.I have a shedful of bits and pieces that follow my experience with hi-fi. I rarely sell anything and love hauling something out to renew my love of them. It's also great finding new combinations. It's not worth many clams but really enjoyable to buy a new item and find the best combination of speakers or cables or amps. If you've got the space and budget keep them and see if something new added to the equation renews the original attraction to them or even sight of them in a new improved light.