If in fact the length of the jumpers makes an audible difference in a given system, I would expect the difference to depend on the impedance of the speakers (especially the impedance at high frequencies), and on whether the cables from the amp are connected to the high frequency terminals, or to the low frequency terminals, or in a diagonal configuration (i.e., one wire to high and one wire to low). And, given the relatively long lengths that were mentioned, on whether or not the two jumpers are twisted together.
For example, the combined inductance of 50 cm and 70 cm conductors of typical gauge, that are not closely spaced such as by twisting, will result in an inductive reactance of around 0.2 ohms at 20 kHz. Some electrostatic speakers, such as many Martin-Logan models, have impedances of 0.4 or 0.5 ohms at 20 kHz. If the speaker cables are connected to the bass terminals on the speakers high frequency currents will be conducted by the jumpers, and in that situation the numbers suggest that a bit of rolloff and phase shifting will occur in the upper treble region.
If the speaker cables are connected to the high frequency terminals in that situation, though, or if the jumpers are twisted together, those effects will be minimized or eliminated and signal transmission to the high frequency section of the speaker will become more accurate (while the inductance of the jumpers won’t affect the low frequency currents they would be conducting). Of course, that improvement in accuracy may or may not be preferable from a subjective standpoint, depending on the speakers, the room, the rest of the system, the source material, and the listener.
As is usual in audio, it depends :-)