I have an all stock 3.8 V6 Firebird and it sounds fine when you punch it. In fact it sound strangely like the revving engine at the start of The Doors ‘LA Woman’. Get a little more air going through the engine and the growl goes up considerably. How many cubes are involved in these faked engine sound cars?
My supercharged 3.8 V6 98 Regal (same basic engine as in your Firebird, IIRC) also has a very satisfying growl when you step on it, which is part of the reason why I have kept it!