I think a lot of people have failed to take one thing into account; the construction methods of older basses and newer basses.
I'll elaborate.
Finishing - Back in the day, most bass builders (Fender and Gibson being the two that immediately come to mind), used very thin coats of nitrocellulose lacquer to finish their basses. Fast forward to today, and most basses are slathered in thick coats of polyurethane, which offers more protection, but also dampens the resonance of the instrument.
Wood - With some species of wood becoming increasingly rare (like mahogany), many non-boutique builders are being forced to use lower quality cuts of wood. Also take into account that way back when, builders were often only using one or two pieces of wood to build a body. Now, you can see upwards of three or four pieces on your average bass body.
Electronics - This is the biggest factor, bar none, as far as I'm concerned. Pretty much every bass, a few decades ago, was passive, save for maybe Alembic. On top of that, pickups were not made like they are today. Most pickups back in the day, were just tossed together - they were scatterwound, by hand, and even used different magnets from pickup to pickup. The old Gibson PAF's are a perfect example of this. I think this is why, you could pick up two identical vintage basses (P's, for example), and one would sound fantastic, while the other might sound average, at best. Today, every pickup is wound exactly the same as the one before it.
I could get deeper into this, but I'm lazy, so I'll leave it at this.
Who knows? I may be waaay off, or I may be right on the mark.