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

Your "bandwidth" and ability to relay that pleasantly continues to awe me ( and I did correctly mean awe, not augh!...)

Thanks for your blurb here...

You're welcome.

BTW: in another forum - if I remember correctly - you said you had these once upon a time: any comments about them?

Yes the L25 Prima was my first real speaker. Well, the I can remember. I forget the Radio Shack ones they replaced. The L25 won out over the Advent as it was much more lifelike (I was in band at the time and hearing live instruments every day) and a lot more efficient. Really wanted the L36 three-way with its nice wood cabinet and midrange but my roommate had a pair so heard a lot of both, the L25 was close and both were better than anything similar in price back then and for many years.

When I built my first DIY speakers, the Roger Sanders transmission line, it was using the 10" woofers from the L25. Their bass response in the TL was impressive. But they were impracticably large for that stage of my life and the woofers went back into the L25s. There they stayed until around 1989 the surrounds began disintegrating.

At this point I made a real blunder. From 1974 to 1989 these JBLs were my reference. In 1974 my reference was real live musical instruments. Trumpet, French horn, sax. By 1989 my reference was L25. Know it now, didn't know it then. Consequently in my mind when I went shopping was that JBL sound. Looking back, I passed on a lot of very good speakers because instead of appreciating what I was hearing I was trying always to match it to my old speakers. The blunder was not realizing this is what was going on. Oh well. Know it now. Live and learn.

As long as you have them, enjoy them. They have endured long enough now to be at least holding their value. But realistically, only to those with an interest in their 70's mod style looks. Or still nostalgic for that JBL sound. I could see a really cool period system with those. If you ever sell, I hope you will take the time to wait for the right buyer. They have endured a long time. Would be cool to see them around a lot more years to come.

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.