Delay is caused by the velocity factor of electricity through the paths it takes. Wire and circuit board traces are generally the fastest parts on the trip, but still you need to take the aggregate length of both routes to know if one is significantly delayed compared to the other. This is easily done with an oscilloscope comparing the output of each path fed by the same input source (i.e. a 1khz sine wave). The best solution is to eliminate the subwoofer altogether and go with speakers that have sufficient bottom end response.
Group delay in subwoofers
I understand the phenomenon of group delay when subwoofers are connected to a preamp and that they need to be placed approximately 8 ft closer to the listener than the mains. Is this also the case when connecting a sub through the speaker level inputs from an amplifier or is group delay much less of an issue in this configuration?