what is the reason having a direct light out of the building if the interior has no windows?
min -4 and max -1 may be considered low for a resolution of 800*600.
the best way to reduce grain from your renders as i use it is to apply linear workflow and under color mapping check the box that says adaptation only while keeping gamma 2.2 and use vray frame buffer to see the actual render while keeping in the rgb space (meaning don't click srgb on the lower right button if you do it will be gamma 4.4).
for that resolution use irradiance map with a high preset and for secondary bounces use light cache at least 1200 subdivs. as for the adaptive dmc settings take your max to 8 and lower the color threshold to 0.005. keep all other settings at their default. lets see what happens.
