So you get better fidelity and clarity and big bass at lower SPLs. A 15' will produce a higher SPL, but 2 8' woofers will be perceived as louder at a lower SPL. To be clear, this is about perceived loudness, not SPL. The math works out so that two loudspeakers close together and in phase are actually four times as effective as one loudspeaker of the same size at low frequencies. 2x8' LF speakers in phase with each other are actually just as effective as a single one of the same total surface area. The 328 will provide better low end than a 312. It depends on your application, but if you want the biggest sound, the 315 will probably be the best choice (especially if you don't want a sub). The 315 is going to be the most 'rockin'' sound.
