The cheap speakers in most MTH trains have limited speaker cone travel.
To get better base, you need a speaker that can accomadate longer travel. These usually have some sort of rolled edge around the speaker cone. MTH has used a thin rolled edge 4 Ohm speaker in some of their recent diesels (MTH Part # BF-0000043.) I have installed these in several diesels and 0-8-0 switchers. Or you can order a high performance 2" speaker from a supplier such as
WWW.MADISOUND.COM. These are approximately 1 3/8" tall and will only fit in some tenders.
For best performance from any speaker, you have to seperate the front sound wave from the rear sound wave in order to keep them from cancelling each other out. This is usually achieved by enclosing the speaker-- the larger the volume of the enclosure, the better. It is nice but not necessary to seal the enclosure.