The most expensive of these is the TX-NR609 which will sell for $599 and boasts THX Select 2 Plus certification.  It boasts Marvell-Qdeo video processing that will upscale your video to 4K resolution.  That's right, 4K.

The $399 TX-NR509 is the step down which integrates the same Audyssey Dynamic Volume and Dynamic EQ as the 609.  It offers 5.1 channels of audio and four HDMI inputs while the 609 offer 7.2 and six respectively.

Onkyo's TX-SR309 ditches Zone 2 functionality and Audyssey EQ features, but still boasts the same TI Burr-Brown DACs as the higher end models.  It also features a front USB port designed to connect with your iPod or iPhone.