The reason why the sound is "flat" is because ideally you should be putting your guitar through an amp, turning the amp up, miking it, running that into a mixer, then into the computer. Hang on, this isn't an ideal world. The best use of amp modelling technology is in the recording area, where you can use something like a POD to effectively cut out all that hassle. Going from guitar to effects and into the computer doesn't help much becaus ethe effects are designed to into an mp that puts the final colouring on the tone.