Matte alpha issue

Actually I’m doing some pruduct viz and I’m facing a boring problem with matte properties.

Please look at images:
A is a render straight from VRay, look at the shadows, they are smooth and not too dark
B is the same scene with the background as matte object for compositing (see attachment for settings). When I paste it onto a plain white, the shadows reveals to be now too dark and noisy …a lot differnt from the original ones.

I think I’ve tryed every combination of the various checkbox but obviously it’s not true; I’ve already test it removing any color mapping values but still no way! …what I am doing wrong?

Thanks in advance