Well, it sounds better; and, although you are wondering why, after all it is physically different, that's to say, the electronics are not the same configuration or same components, so would not a different sound actually be expected. Only thing that would make me wonder would be if you replaced the PM8005 with another PM8005 and you perceived one sounded different than the other for better or worse. One thing though, you might want to get an external crossover to set your mains at a cut off at about 60hz. Seems to me that would get a better result, or just turn off the sub.