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

Computer programs that accurately model the bass response of loudspeakers do not use cone diameter for anything other than calculating maximum SPL.

Exactly - that is where large woofers or several large woofers become important. And at low frequencies where hearing is much less sensitive it is generally obligatory to have very large woofer(s) in order to achieve SPL's greater than 100 db at 20Hz with minimal distortion. However, in a good design, with a high quality driver, a 6 inch woofer can do a pretty darn good job down to about 60 Hz - just don't expect miracles that defy the laws of physics. (Anyone with 6 inch woofers on their main speakers must have seen the remarkable excursions that they do when playing bass heavy music and these large excursions means that a fair portion of what you hear is distortion)

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

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.

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.

quality VS quantity. The eternal struggle.

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

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.

Has,
Please 'do the math'.
Double the diameter of a circle and the area goes up 4X.

This means that 2x6" drivers have half the area of a single 12.

The main thing I believe is,does the sub woofer have the sound pressure to match the speakers? What I'm trying to say is,if your playing your speakers at the loudest level you like,will the sub produce the low frequencies you like at the same level?And do so without distortion or stress?If it does I don't see why not.

Your sub is a Transmission Line design. The size of the drivers being 6 1/2 inches is less important than the design of the box they mount into. The box itself is tuned to play that low in frequency by the length of the line built within them using those specific drivers. Two six inch drivers will move a consciderable amount of air , much like a single 12 inch would. Focal use to sell 4 inch subs that played ridiculously low and punchy. The size of the driver in subs depends on the quality and design ( sealed, ported, TLE) of the sub enclosure. Large drivers do not dictate the quality of bass you hear in either 2 channel or home theatre. Cheers