The PB12 has a bump in the frequency response around 50 - 60hz, which I found a little annoying as it made the sub sound more like a car subwoofer. The EP500 has DSP that smoothes out the frequency response for a linear/flat playback, just nice and smooth sound reproduction through the range. The DSP also limits the output making port noise nearly nonexistant on the 500,

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70