Dheim speaks the truth.
I use double screw or allen as my default bridge config. It increases the bass, all things being equal, maybe 15% or so, as a hand-waving guess. The biggest thing is you can use the relative height of the neckwards and bridgewards coils as a clumsy EQ - neckwards coils higher, more bass, and vice versa.
Allen poles give less dispersal of the magnetic field and so direct it more strongly at the string = slightly more focussed tone and more attack, itsy bit more high end.