I suspect that somewhere along the way, JBL accidentally mixed up their plate amp photos. I do not believe the 550p has speaker level inputs. If you look at the Amazon listing, you will see the speaker level inputs. Currently there are eight 5-star reviews and two 2-star (or 1-star?) reviews. The 2 "negative" reviews are by people who bought this sub with the expectation/need for speaker level inputs!
Like you, I am disappointed that JBL didn't "grab he bull by the horns" and start producing high performance subwoofers. However, my expectations for a 10" sealed sub are a good woofer to fill out the low frequencies for music, and that falls squarely within JBL's competencies! Anyone looking for LFE should look elsewhere.