The impedance rating of a loudspeaker is a difficult thing to bring down to a single number like 8 ohms or 4 ohms. The impedance of a loudspeaker varies widely with every frequency. The minimum impedance along the entire frequency range is probably the most important number but what frequency this minimum impedance occurs at would be equally as important. All this needs to translate back to the real world volume level the amplifier may go into protection mode at, which is all that matters in your listening experience. Add to this that different amplifiers are more sensitive to lower impedances and it gets even more confusing. As for your situation you will not have any problem with the VP150, as far as 4 ohm rated speakers go the VP150 is a very easy load for the amplifier, probably easier than the majority of 8 ohm rated speakers on the market.
Ian Colquhoun
President & Chief Engineer
