You don't take the back off!! Either unscrew or prise off the front grille (if there is one) and simply unscrew the bass driver - that is your way in. Okay, with some you need to be a gyneacologist especially with Bose 802s... but conventional woofer/tweeter cabs are easy enough.

In some instances the grille (round) may be held on by the bass driver retaining screws, some are held on by Velcro, and some (particularly in high-end such as some Bose and JBL) are a simple interference fit.

They are made this way for ease of manufacture - a separate back takes more time to construct, and can also give rise to unwanted rattles. It's also much easier to cover the cabinet in Rexine or Carpet.