I always thought the ambient term has to only be added to the diffuse/specular light.
edit: Actually no, if I add it to the albedo*lighting I get a weird merged ssao (white/grey) and albedo picture.
But anyway the AO term isn't the problem.

I guess showing it on pictures is easier:

Note: The blurred images already include tonemapping and BG lighting is turned down (0.2f)

Lighing only (blur off): http://cl.ly/image/3x0o1w451W2x
Compose (blur off): http://cl.ly/image/3K022S2a2Y0g

Lighting only (4x blur): http://cl.ly/image/0g0q0z1n0937
Compose (4x blur): http://cl.ly/image/3D2j0K04413Y

As you can also see the back of the cube is leaking the light from the background, too which is a problem...

sorry my fault, you should actually modulate with the SSAO term. Try:

output = AO*albedo * float4(lighting.rgb, 1.0f);

But then I don't know very well how you structured your pipeline, so If there is a problem elsewhere It would probably influence your results...