Boom is EQ software - but there's only so much an EQ is going to do for the tiny loudspeakers built into laptops. Often, a "full-range" driver inside a laptop is about the size of a traditional loudspeaker's tweeter. There's only so much dynamic range you're going to get out of something that small.
If you truly want better sound, you'll want to connect external speakers.
Regarding how your tinny sound remains after uninstalling Boom, I would imagine it's leaving something behind that's affecting the settings (perhaps a .plist file somewhere?). Not quite sure there. 😛