I suggest you go back to the orginal method, and store the greyscale detail maps in the alpha channel of your 32 bit tga files. This way you could even have up to 3 detail maps, one for each texture blended on the terrain...this basically gives you three textures for the price of none, as you have already done the float4 reads

you access the alpha channel in the shader like this:
FinalColor.rgb*=RedColor.a