Efficiency (1w/1m) is not an accurate indicator of a subwoofer's output capability and should not be used as a comparison to other speakers or subwoofers to determine which one is "louder".
Round/Circle: This is the typical and common subwoofer design. They have a round cone and are a circle in design.
Square: These woofers are able to achieve a greater cone area in a smaller space. Because of the greater surface area, some square subwoofers will be louder than a round subwoofer of the same size.
Recommended Enclosure Specifications
Sealed: These enclosures completely enclose the air inside the box. These boxes produce a tight bass.
Ported/Vented: These enclosures have a slot of air to escape from the box. The result is a sound that has more "boom".
Bandpass: These are a combination of both sealed and ported enclosures. They can be very efficient, but require tuning for the best response.
Shallow mount speakers will typically have a mounting depth of 2" or less, while shallow mount subwoofers will typically have a mounting depth of 3" or less.