The sub up front effect is based on phsyics. However, your wrong in regards to the fact that phsyics isn't the only thing dictating how a sub works, because it is. It's simply too complex to calculate for all parameters. However, on one level your correct, physics is an attempt to describe reality, not the other way around.
Frequencies below 80hz or so, are phsically longer than the car. Becuase of this, you cannot locate the souce of the sound. The wave wont' be within a substantially differenent position no matter where it is. The wave will be either postive and negative on both sides of your head at the same time. Becuase of this, your ears have no phsyical way to tell the difference. Due to the actual nature of this phenonmenon, the bigger your car, theoretically the higher your xo point can be. A rattling panel can quickly ruin this effect though.
The 250hz number, is slightly different (unless your car is 100feet long, then it'll be fine too). Below 300hz or so, a drivers on axis response and off axis response are the same, however, it is localizeable.