He wishes to get rid of the higher frequencies (that will required a tuned baffled system to cancel out the higher frequencies selectively, or a material that absorbs the higher frequencies only), that makes the exhausts sound tinny. Additionally he wishes to prevent droning which is a problem when the car cabin becomes a resonance chamber reinforcing (rather than canceling out) the harmonics in the exhaust sound.
There are materials that can be selected to absorb a specified acoustical signal (sound frequency, but you have to select a material that is also immune to high heat, moisture and the acids found in the exhaust pipe. This limits the material list and if you include cost that is not going to result in an economical solution.
Acoustics has advanced to the point where we can engineer a simple baffled muffle that will cancel any specified frequency. All you have to do is set up a steel walls to reflect the sound back at the source a specified distance from the source. The distances varies with the frequency you wish to cancel.
Since sound has a near infinite number of frequencies (though not all are generated in a car's exhaust), you will need a bunch of reflecting surfaces to reflect the sound back at the source. An alternative is to set your reflecting surface at an angle to reflect it at another angled surface and let the bouncing sound waves cancel each other as they are reflected and absorbed dissipating their acoustical energy. The more room inside the echo chamber and the more reflecting walls the quieter the end result will be.
Stock exhaust systems use both systems to dump the expanding and cooling exhaust into two to four chambers that vary in size connected by perforated pipe that has different sized holes in the pipe to allow the various frequencies to be absorbed and canceled out by echoing in the different sized chambers.
This what a Flowmaster 40 series looks like inside:
Here is what a Flomaster 70 series looks like inside. The 70 series is a longer and wider muffler than the smaller 40 series so it is harder to find room for it under a smaller car:
Though a Magnaflow looks like a baffled exhaust on the outside (due to it's oval shaped case) it is basically a glass pack that is generally round in shape:
The inside of a typical Thrush glass pack:
A close up inside a conventional muffler showing how the gas gets out of the pipe to cool and expand absorbing energy:
A diagram showing the interior of the of a Walker muffler: