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? My understanding was that both response curves were pretty flat within the frequency range of the VP150, but outside that range (ie lower frequencies) the VP180 was still pretty flat while the VP150 was... um... less flat 
M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8
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