I'd agree. My cabs had substantial bracing, and were constructed from 1" MDF with a 2" MDF sandwich for the front baffle. Here's an early picture showing the bracing, which is recessed 6mm into grooves in the main cabinet walls.

These will not win any prizes for ultimate extension- they were originally designed for use in a Tact controlled sub/sat set up crossed over to the mains at 350hz. Low distortion  was therefore of more importance. For a pure subwoofer, I would use a very different drive unit. I did play with the radial volt in the same cab and it's a punchy little sod! Again though, for pure sub bass, it really doesn't cut it IMHO.