Jeremy - when it comes to speakers you really have two options: 1.) fit in the stock space, or 2.) modify the model to fit it somewhere else.
I've messed around with speakers quite a bit. I used to make my own enclosures for sugar cubes, and would install 2 or 4 depending on space. At first I thought that they sounded spectacular. Then I experimented with Scale Sound Systems speakers. I quickly learned that my homemade speakers were vastly inferior to the SSS speakers. For ease of use and installation, the SSS speakers can't be beat. However, I liked their older speakers much more than their new ones. The old speakers could be ordered with two speakers installed in one enclosure, providing more and richer sound. The new ones are a single speaker and they just aren't as good as the old. Much better than most everything else, but I miss the old ones.
If you want bass, then really the only option is a speaker module from TB speakers. They come as a module with a decent sized driver and a passive radiator. These two combined produce low notes very well. They don't get nearly as loud as other speakers, but they also don't sound like nails on a chalkboard. Listening to most model railroad speakers is a form voluntary torture. The TB speakers are the only ones that can actually reproduce low notes....and paired with Railpro's newer sounds they are quite excellent.
The downside is the size. The only way to fit them is with some serious surgery to the locomotive. Large amount of the weight needs to be removed, and you need to be comfortable with soldering and running wires. Is it worth it? Yes. I have a few models with the TB speakers and they hand's down the best sounding models that I have. Not annoying to listen to. They sound about as good as you can get in HO scale. This video shows the installation in one of my models:
-Kevin