You're right that modulator output is amplified already (don't remember the technical term for this). Some distribution amplifiers have special inputs for modulators to take this into account. If you use an amplifier without this feature, you can put an attenuator in between the output of your modulator and the input of the amplifier. This will avoid overamplification of the signal, which messes it up.
Radio Shack sells a variable attenuator (with a dial to control level of attenuation). I used this & it has worked well.
I'm not sure about the Channel Vision products. I would suggest that you check with their technical support to clarify. But, if their products are similar to Channel Plus, then the modulator inputs are NOT amplified, because they assume that you are using a modulator which already supplies an amplified signal.