I think some classes of loco work better than others. It depends on what (to my ear anyway) is the main characteristic. For instance the EE type 2 (class 20) works well as its main sound characteristic (as we all know) is it's whistling turbo. Others, such as the Deltic have a really bassy main sound (apart from the clatter of all those meshing gears) so it doesn't work as well through a small loco-fitted speaker. The "throb" of a 56 is another one that could really do with a decently large speaker as are most steam locos where the high frequency needed for steam lends itself to the small loco speaker but the "chuff" really needs something bigger
