Normal Practice To Use 6 1/2" Sub With 8" Speaker?

Large drivers do not dictate the quality of bass you hear in either 2 channel or home theatre

Perhaps not but they sure determine the quantity of good quality bass. Only large drivers (several of them) will get you up into the 110 db SPL at 20 Hz with low distortion.

quality VS quantity. The eternal struggle.

But of course, as the Russians (military) say, 'Quantity has a quality all its own'.

Well two 6 six inch drivers have the same surface area of one 8.46 inch driver.

Anyway in the end it is all about the sound and it should sound fine as long as you don't push the SPL.

I upgraded from a single 12" Dali sub to dual 6 1/2" PMC sub. The bass quality of the PMC is better and goes lower (22Hz) than the Dali (28Hz) on paper...something which I fail to understand why a single 12" driver fails to go lower than an equivalent 9.12"(2x6.5") driver. The PMC retails at $3,000 while the Dali at $600 though, if that tells anything.

Ryder, woofer diameter is not a reliable predictor of bass extension. Often designers use large-diameter cones to reproduce very deep bass because a lot of air movement is required, but the cone size itself is not what causes a speaker to have deep bass extension (though along with excursion and enclosure design, it is a factor in how much air a woofer can effectively move). Many factors come into play, with cone diameter contributing to one of those factors directly, and to another indirectly. But the motor, suspension system, and enclosure design all matter more than the cone diameter. Computer programs that accurately model the bass response of loudspeakers do not use cone diameter for anything other than calculating maximum SPL.

By way of analogy, tire diameter is easy to see, but doesn't reliably predict the performance of an automobile. The motor, suspension system, and body/chassis design all matter more.

Duke
dealer/manufacturer