Scale length, as I've found through my research and experiences (search on threads titled "Stambaugh Twins") and you'll see I've played basses that were virtually identical in all respects, except for scale length.
To begin with, I currently own two basses that have a low B string, one's a Lakland DJ5, a 35" scale, the other is a Jerzy Drozd Mastery VI, a 34" scale. Which would YOU think (on paper) has a tighter low B, especially given Lakland's very well known tight B?
Actually, the Jerzy has a better B than the Lakland. It's also easier to play because the slight increase in scale length adds to additional string tension (again, see my thread about the Stambaugh Twins) and stretching more in the first position.
I have very large hands and don't have a problem playing the Lakland, but the Jerzy, even with an extra string, is simply a better playing instrument, and has a better B, to boot.
On the other side of the debate, the Stambaugh Twins I played were both neck-through, both made with the same woods, same electronics, hardware, etc. One was 35", the other was 34". The 35" DID have a better B, but was also less comfortable to play. The 34" played like butter, but the B was passable. Chris did build this with body-through stringing, which I've since played with, and found it tightens up the B a bit.
The point I'm trying to make is, if you make it truly scientific, you're missing out on a world of talented luthiers who can eek out magic from seemingly run-of-the-mill materials (can you tell I'm an enormous Jerzy Drozd fan???). BTW: I've also played 35" Jerzys, since that's what he tends to build, and the B isn't really any better than on my 34" Jerzy.
An interesting point I want to highlight: I was hanging out at LAG one day and a guy came in who had tiny hands. He had Keith Roscoe custom build him a 33" Roscoe, which apparently Keith was pretty much against from the outset. I got a chance to tinker with this 33" Roscoe, as it was being considered for a trade with LAG for a Jerzy. The 33" Roscoe had a tremendous low B, one I'd put against any of the best out there.