I picked up some cheap 3/4" speakers from ebay (about $1 each?) that were listed as having a good bass output. The sound was 'meh', however after searching around a bit I learned something -- you need a baffle behind the speaker to get good quality sound. I 3D-printed some to fit what I have, but essentially it is a sealed plastic can where the speaker fits into the top, and it forces all of the sound to come out the front side. And I mean ALL of the sound. I have a Bachmann 4-6-0 that I put one in, connected to a tsunami2. I haven't even drilled any holes in the tender but the sound is so loud that I had to turn down the volume to about 200.
As for the sound quality, I can hear a slightly tinny tone to some sounds such as when they're shoveling coal and in the higher tones of steam during the chuffs. But at the same time I am getting an extremely surprising amount of bass out of this setup for the small size of the speaker, and I haven't heard anything better at the train shows. SoundTraxx and others sell speaker baffle kits, so if you're not able to 3D print your own then I suggest buying a kit and see what you think of the sound quality before investing in high-cost speakers.
What a lot of people don't understand about speakers is that the cone sends out sound waves as it moves away from the magnet and as it is pulled back to the magnet. Without a baffle, these two sound waves basically cancel each other out. With a good airtight baffle, only the sound waves from one side of the cone are heard. This is why a baffle dramatically increases the volume of the sound.