It looks like the 2809 is newer and has most of the bells and whistles, it replaced the 2808.
From the Denon website.
For a truly stunning surround sound experience, the AVR-2809CI is packed with the latest audio and video technologies, for the ultimate performance with next-generation formats such as Blu-ray. The advanced surround sound processing section is powered by two 32 bit floating point DSP chips, and features the highest resolution surround decoding, including Dolby TrueHD, Dolby Digital Plus, DTS-HD Master Audio, along with a full suite of additional surround decoding functions. The powerful seven channel amplifier section features 115 watts of power per channel, and provides the choice of driving a 5.1 channel system in the main room, with another stereo speaker pair in another room with independent source selection. The advanced video section features four v1.3a HDMI inputs, along with four additional wideband component video inputs, with Faroudja DCDi video processing and up-conversion that allows a single component or HDMI cable run to the HDTV. The AVR-2809CI is one of the first to feature Audyssey Dynamic Volume, which automatically eliminates irritating jumps in volume between TV shows and commercials, as well as Audyssey Dynamic EQ to ensure rich bass and smooth tonal balance, even at low volume levels. For optimum audio fidelity, the Audyssey MultEQ in-room acoustic correction system measures the overall room response at up to eight measurement points and tailors the sound for the most natural tonal balance everywhere in the listening space.
