For the most part a larger chamber will lower the system fb and that gives a better bottom, while also serving as a high pass filter to keep unwanted midbass and higher frequencies out of the horn. But as in all things you can overdo it. Most of the peaks you see in McBean don't turn out nearly so bad in the actual product.
Do not put any damping material in the horn; damping should only go in the chamber, and that should be fully lined to prevent any midrange reflections back to the driver cone. Damping within the horn is only required with 'quasi' horns, those that don't have a constantly expanding taper but are in fact a series of ever larger constant cross-section pipes placed end to end. These aren't true horns, and in my opinion should be avoided.