The efficiency of the speaker and a proper baffle will go a long way to making better and louder sound. Just by dropping a good baffle over an existing speaker, you can make a dramatic change in both the volume and bass response of most speakers.
FWIW, I will add the following. I've recently been modifying one of the recent Legacy E7 SuperBass B units to put it in an E8/9 MTH B unit shell. I don't think that these latest engines have quite the booming sounds of some earlier SuperBass units, although whether that's a function of the sound file, amp specs or speaker/baffle size I can't tell.
Anyway, to adjust for different placement of the smoke unit outlets, I had to modify the stock baffle/speaker enclosure. These are pretty odd looking (see photo, which shows current E7 in the back and an older, more squared off, SuperBass one in front). They are slightly saxophone- or maybe trombone-shaped, and I don't know exactly what the theory behind them is. However, shortening the taller part of the E7 enclosure (in order to get the smoke unit placed correctly relative to the outlets) made no significant difference to the sound; but not having it at all, or substituting a much smaller/flatter enclosure, certainly did.
The flatter enclosures are fairly typical of what I have seen in Lionel diesels and indeed steamers that have two speakers. I have only really achieved an increase in the depth/resonance of sound by putting in a bigger enclosure. As people have noted here before, the round plastic top of a rattle paint can can be used for this purpose if you have room in a tender to do so. The absence of any baffle makes a huge difference.