I am using a software modem. How well does TweakMASTER work for this setup?

Once upon a time there were true "hardware" modems… Nowadays, most internal modems are very "dumb", and most of their functionality is in the driver. This allows manufacturers to cut costs. Of course, it slows down the whole computer, and doesn't work on any operating system except Windows. These are what are referred to as "software", or "Windows-only" modems. As far as TweakMASTER is concerned, there's no difference. In general, software modems perform worse than true hardware ones.