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

What douglas_schroeder said, which should be obvious. I think you would be hard pressed to find very many pieces of audio equipment that are worth more today than when new when inflation is taken into account. There will always be exceptions to the rule.

It's funny some do and some don't. 

My fellow Asian folk will always more for well respected Jbl or Tannoy , altec Lansing driver , for that kind of moneys they spend I rather pay for recently made and still have moneys left for more records 

This hobby doesn't seem to have as many "collectors" as other expensive male hobbies. People want their money to buy great sound above all else. Classic cars, this hobby ain't. 

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.