Sat,as you know,subs and all other speakers work primarily against the air OUTside the enclosure. Ported designs also give the air in the port an active role. Although the majority of speakers are ported, this certainly isn't essential and there are many sealed speakers, including your BPS150. Mirage is big on the "bipolar" theme, but here all it means is two drivers instead of one in a sealed enclosure. Since the opposite movements of the drivers tend to cancel out the enclosure vibration, I suppose that your sub won't "go walking".
