The Teac AG-L800 uses tripath output devices for each channel instead of a standard class A/B design in the AG-H550. It's a form of a class D amplifier with a very high efficiency and low distortion. The more efficient design allows the receiver to be smaller than the similarly performing AG-H550. Other than preouts, the AG-L800 has virtually everything the AG-H550 has, minus 12 pounds from the more efficient design. The AG-L800 is newer and includes Dolby Pro Logic II, the AG-H550 sports the standard Pro Logic.
For looks I prefer the AG-H550, even though it is larger overall. I've never had one and am still on the hunt (short of the A-H500 integrated, it's at the top of my list of things to get.
The AG-L800 seems to have more people interested in it than the AG-H550 for the tripath amplifier technology inside. DIY'ers and audiophiles modify the Teac tripath based products as well. This trend was set when a company named "Sonic Impact" released a little plastic 15 watt per channel amplifier for dirt cheap and audiophiles just soaked that little thing up. Since then, anything and everything tripath has become an "audiophile secret" or something. I'll admit, this thing sounds very refined for a receiver it's weight and dimensions. I still prefer the look and overall operation of my AG-H500 stereo receiver. The AG-L800 just looks funny sitting on top of the PD-H570 7 CD changer.