[font=trebuchet ms]At least part of the problem is that a guitar amp is intended to add pleasing kinds of distortion to the signal applied to it from a guitar. In order to reproduce the sound of your effect box, you need an amp that is accurate, which is not the same thing at all.

Another possible problem is the issue of signal level. The output of your effect unit may not be as strong as the signal from your guitar, so the amp isn't being driven as hard. Is the effect output adjustable? Can you turn it up instead of the amp?[/font]