The stylus is the actual "needle". The cartridge is the entire square thingee. I never replace the stylus, just the cartridge after 1,000 hours or so.
Before you spend anything, of course make sure that any other source (CD, digital) doesn't have that same muddy bass. Also, make sure it's not the album - for example, side 4 of the "blue" beatles greatest hits is very bass heavy for some reason. At least on my system. But it's somewhat isolated to that.
Lastly, make sure the cartridge is aligned properly. I'm horrible at this, and hate doing it.
I use
this cartridge.