The second version looks better -- it's added smatterings of dirt at random spots. Yes, it does darken the map a bit; that's increased by the fact that you're layering your grunge onto a fairly dark texture to begin with. If you want it lighter, try adding an adjustment layer in between the tile layer and the grunge layer to bring up the lightness of the pattern before the grunge is layered onto it. Curves, Brightness/Contrast, or Hue/Saturation can all be made to work for this -- pick one and experiment.

Alternatively, try adding a layer mask to the grunge effect, then use a soft black brush on the mask to hide the grunge in areas that you want brighter. You can, for example, leave dark, heavy grunge around the edges of the room while erasing it from the middle. Or you can leave it in most places, but carve out "clean" paths where people actively walk.

It's a bit easier when you have grunge going onto a lighter background. Here's some sandy soil with some grunge:

Click image for larger version.  Name: grungy-dirt.jpg  Views: 40  Size: 1.14 MB  ID: 86005

Specifically, this is a 2000x2000 layer of Soil Sand 18 with Grunge Map 154 layered on top of it at 50% opacity. The grunge does two things:

1) It makes it much less obvious that this is a repeating, tiled pattern by breaking up repetitions.

2) The tonal variation suggests minor changes in height (darker = lower), or possibly in the soil's water content (darker = muddy).

I really like the grunge technique for soil, grass, and other organic things which are not supposed to have a repeating pattern to them. It's not quite as good for things like brick and tile which are supposed to have repeating patterns normally. In those cases I pick a dark grunge image, but turn the opacity down really low -- the idea being to get some subtle tonal variation without losing the basic pattern.