Beg to differ, I was in the retail and professional audio contracting for many years; sub-woofers whether 10", 12" or 15" are optimize loudspeakers and enclosures to reproduce extremely low frequencies less than 100 Hz. Full range systems are designed for a much wider range and are normally deficient in the extreme low end.
All individual loudspeakers have what is call their "free air resonance" the smaller and/or stiffer cones have a higher FAR, acoustic suspension woofers rely on the air in a sealed enclosure to provide the force to control the cone, a great way to get low bass out of a bookshelf size speaker. Henry Kloss designed the first acoustic suspension speakers by Acoustic Research, then KLH, and finally Advent. Most woofers are optimized by "tuning" the enclosure with a bass port to flatten out the peak at the free air resonance point. For large systems the woofer can loaded by a front or rear horn design to increase efficiency but low end suffers.
I am sure Apple has done all it can to optimize it's 4" woofer in the HP, but just isn't going to reproduce fundamental bass.