Chris, as you know, speaker impedance commonly varies significantly with frequency, and usually the manufacturer rates it at an average figure(the M80 is an exception, since its 8 ohm nature over the majority of the frequency range would justify a 6 ohm average rating). This may simply be a determination that the average for the VP150 is more realistically 8 rather than 6 ohms and that the 6 ohm rating may be confusing to some users who think in terms of 8 and 4 ohms. It seems unlikely that there was actually a change in the drivers so that their impedance was increased. In any case, the difference is nothing to be concerned about.
