The main difference between the receivers is power.
The Concert AVR-7 outputs 100 watts per channel into 8 ohms. While it will play into 4 ohms, it is a Class AB regulated power supply so the power output will not increase when it sees a lower impedance.
The Concert AVR-9 uses a high current Class H design which outputs 120 watts per channel into 8 ohms and 200 watts per channel in to 4 ohms; So, it will send more power when it sees a lower impedance, much like our stand alone amplifier the Savoy G3.
Concert AVR product pages here