A six inch diameter speaker will be smaller than the wavelength (actually a quarter wave I think) below 500 Hz or so. Below that frequency the speaker will output mostly pure sine waves. Like all speakers they will have 2nd or 3rd harmonic distortion which is usually down more than 30 dB at reasonable volume for either round or oval.

The speaker specifications should have the Thielle/Small parameters, they will include X-max (maximum cone displacement) and Fs (speaker self resonance). Speakers have very little useful output lower than the Fs frequency. Note that a cabinet or baffle can influence the Fs spec.

Since you are essentially modeling a woofer many free woofer enclosure design programs are on the net. They will usually model X-max vs input power or output SPL.

The volumes of the individual speaker chambers look a little small to me for efficient 70Hz performance.