No Speakers!!!

I was trying out the sound in my new 94z28 (bose) and noticed that it sounded great at low volumes, but then the Lower frequencies cut out once you turn it up!
so i popped the back speaker covers off and THERE ARE NO BACK SEAT SPEAKERS AT ALL...
Kind of crappy... they missed a couple of light bulbs in my other camaro, but two whole speakers????

so, which speakers do i get for the car..???
all i know about the stereo is that it's delco/bose.

The Bose system had 2 speaker options from the factory. The 3 speaker configuration has the 2 doors and the sub. The 5 speaker upgrade added the rear seat speakers.

Can I upgrade to the 5?
and say, if the lower frequencies cut out at higher volumes, that WOULDN"T mean that the sub's BAD right? it just can't take all of it itself....

94 Zs only came with the 3 speaker setup. I'm not sure what model year the 5 speaker system came into use, but it was never an upgrade to the 3 speaker system. It was either base radio or that year's Bose. You would have to pull an entire system out of a newer car to upgrade.
If you're missing lows at high volumes, it may be one of your amp's going bad. I finally pulled my Bose and replaced it, as the 9 yr old system was dying a slow death.