I had mine upgraded a couple of months ago and as far as mine goes I think the picture quality is better than the old HD box.
Do you have both the Scart and HDMI connections joined up? If so it might be worth disconnecting the Scart lead. I had both connected and found that my TV defaulted to the Scart connection at each switch on and I didn't always notice it.
If this doesn't solve the problem get VM back to have the box replaced. There are faulty boxes. When I had mine installed the first one the engineer put in (brand new out of the box) wouldn't work at all and he had to change it for another one.