On the other hand, if the frequency response curve is ruler flat for the VP150 and also for the VP180, why do they sound different at normal listening levels? Could it be that frequency response curves are not really determinative of a speaker's sound quality?
Actually the FR curves do tell about a speakers SQ, if the FR is not linear or near flat then you can be assured of hearing a slightly (or worse) colored sound. The M3 is a perfect example, It has a different sound to it than the M22s and the M3's FR shows that difference, a dip in the lower midrange and a slightly elevated upper bass hump.
Having said that, FR curves are not the end all be all of how a speaker will sound, as has been mentioned, there are many other factors involved and the absolute worst factor is the listener's own preferences that can not be accounted for via a FR graph. But the FR graph will give you an idea of how much the basic speaker design is influencing what you are hearing.