I think/guess most often they're sealed but that's a good question and we deserve some answers !!! ... okay, too much coffee.
YAA, I liked the Phil3s in stereo better than M/C w/ the SC. Not a big difference but they just had this slightly more natural sound with vocals that I think audiophile/BS'ers might refer to as 'revealing'. Words like subtleties and nuance need to be used in this proclamation by somebody with a better working knowledge of English.
I take it the Phil center has bridged that gap and, and, and ... just maybe ... exceeded the listening bliss offered by the Phil3s alone? Oh, yeah ... Adam is absolutely correct ... you suck.