A good subwoofer should have at least the following:
- High SPL
- Low distortion, proper cabinet, shorting rings in the motor, etc.
- Low Q, no peaks, proper cabinet size, etc.
- Low port noise
- Protection from overload, amp, bellow tuning, etc.
- Sturdy cabinet, no vibrations, rattling noises, etc.
- Powerful enough amplifier, no overheating.
Am I missing anything else? The subwoofers listed do all of the above and do it better than anyone else.
If we're talking purely cosmetics, or size limitations, then sure: Velodyne, JL, Paradigm, etc. would all qualify. But this list is purely for performance only.