What is the difference between SQ and SPL subwoofers?

SPL subs usually have a high Fs (hz). This is because it is easier to get high SPL at a higher frequency. SQ subs usually have a low Fs, because they go lower. QTS is usually an indicater of what kind of an enclosure a sub is partial to. (.7) Qts means the sub should go in a sealed enclosure. A sealed enclosure is thought to be the "smoothest" type. It doesn't have a hard roll-off on the upper or lower end of the frequency response. The size of the structure can also be a consideration. As with anything, the bigger it is, the harder it is to control. SQ subs will usually not have huge magnet and voice coil structres or extreme excursion (cone travel). Some good subs that combine SQ and SPL are OZ (as you stated), Diamond Audio (D9), Treo (TSi), and JL Audio (W6V2)

If you enjoy quality SQ. If you enjoy loudness SPl.

A sound quality driver has higher expectations placed upon it because it is made of better sounding materials and is expected to help make us believe in a higher authority when installed in the right box with the proper power. When consumers and dealers search for a SPL driver they would readily accept a woofer with a cone made of recycled aluminum cans and used hypodermic needles if it had a large enough motor structure and was LOUD. Ask yourself what defines a SPL woofer; is it cosmetics, shipping weight, chrome, 16 spiders, giant belt buckle bending motors, or output. Hope this is helpful.