You might as well continue on YouTube looking for a pre-1970 Starfire demo.
I would describe the humbuckers as "muddy" at the low end. In a bass amp with every available control on the amp and bass set to maximize bass and minimize treble the result is an indistinct "thump" at an almost un-discernible pitch. That was my go to tone in cases where I had no clue what I was doing because no one really knew whether I played a wrong note or not. Moving away from the extreme I'm not sure I have words to describe the tone. I can tell you what it doesn't sound like (a Bisonic or a P or a J) but not what it does.
You need to take my opinion with an entire shaker of salt. I have only heard the humbuckers in a solid body and some of my Bisonic comparisons have shown me that there is a "woody" component to the sound that I like that comes from being in a hollow body (vs. solid) and not necessarily from the PU. The "deep/hard" switch is still intact and in the circuit in my humbucker equipped bass. People who have rewired have been very impressed with the results.
If you are looking for buying advice, any Starfire is better than none. Beyond that, it's up to you. Deciding whether you need one or two pickups (or don't care) effects your choices. "Minty" effects your budget. For example, if sound trumped minty then I might spend $2300 on a vintage Starfire I that has seen better days but had the vintage Bisonic intact. At some price point, the Newark Street basses become viable candidates.
I have a vague recollection that someone who had a 70's Starfire with humbuckers, really, really liked it, but they were also explicit about not striving for or caring about the "Bisonic" tone and Jack Casady and Phil Lesh were not among their tone heroes.