These are good speakers what that money can buy. I happen to own one of their models myself, and it continues to do its job really well.
Regarding your concern, Let's delve into the technicalities regarding impedance right?. The impedance value you've cited is nominal, meaning it serves as an average representation rather than a fixed value. Consequently, it doesn't carry as much significance for your Audio Video Receiver (AVR) as one might assume.
The concern regarding speaker impedance primarily arises when you opt not to incorporate a subwoofer, thus requiring your Left, Center, and Right (LCR) speakers to handle the entire frequency spectrum. In such a scenario, impedance becomes a critical consideration.
It's crucial to understand that impedance only becomes a significant factor when all channels are operational simultaneously and drawing significant power. Even premium AVRs like the Denon 4700/4800, can experience clipping under high dynamic load conditions, particularly during intense movie scenes played at elevated volumes within a well-treated home theater environment if there are no subwoofers at play.
However, introducing a subwoofer into the setup and configuring the LCR speakers as "Small" with a crossover (XO) frequency set above 80Hz mitigates these concerns. In this configuration, the impedance of the speakers, whether 4 ohms or 8 ohms, becomes less consequential.
So i'd say, if your budget does not permit to go higher, at least get the 1700 and not the 550BT.
Regards,
Som