Ah, here's something I have discovered.

Although some of the diffuse on the floors are still the same as the first render, in this latest render, the diffuse on the colums seem to be grainy, but in a tiny way. I take that as a good omen.

MinRad was 0.01

But I suspect that maxrad has some major role to play here, I'll do another render, starting with minrad back at 1 and decrease the maxrad, like I did on the minrad setting.