Interesting thread.  I recently purchased a 70s Series I and my first impression was that it sounded more open when compared to my 2005 Orion, which I felt had a deeper, richer, and/or smoother tone. I'm using Pyramid Gold flat wounds on both basses.  From what I understand, the Series I had pickup upgrades sometime in the 80s. Although, I guess there are a variety of factors that could contribute to this apparent sound difference, including wood combinations.

In addition to the aforementioned Series I, I have a 2000 Orion very similar to yours, rv_bass. Probably the two biggest reasons for the differences are Activator (Orion) vs. Filter (Series) electronics. And then set-neck (Orion) vs. neck-through (Series).

Pickup location also is a big influence on the sound of the bass. Something Ive noticed on my own basses of the same design.

Another observation. The new pickups are MUCH louder.

Here's a sample of the tone. I chose a ballad because you can hear the bass better between all the other instruments. We hadn't played this one in about 6 months, so there are a fair number of clunkers.
 
https://www.dropbox.com/s/vc2qgvuk67asaf5/Black%20Peter.mp3?dl=0
 
Just to compare to the old ones (of course, there are other variables, including the DI):
 
https://www.dropbox.com/s/mh4zrbj2c37gwq1/Row%20Jimmy_1.mp3?dl=0

I can record a comparison between a 1979 and a 2015 Series I pickup, in the same bass with the exact same settings. (only neckpickup btw, the bridgepickup is soldered to the preampcard)
 
I didn't notice any radical changes after replacing the pickup (apart from the corrected phase issue of course)
 
Not sure if I can do it this year, most likely somewhere next week. :-)

By 1979 I think that most of the specs of the pickups were pretty much solidified. This is a comparison between a set of pickups that I gauge to be ca. 1973, have trapezoidal magnets, and a resistance of about 800 ohms, vs. 2015 pickups.  
 
I don't think you'd hear much difference between 1979 and 2015 pickups. Just saying.