For example, one (of many) causes of tinnitus is otosclerosis--the "stirrup" bone in your ear grows extra bone and becomes unable to vibrate properly. The primary symptom of this problem is somewhat muffled hearing, but it is often accompanied by tinnitus.
This condition comes on extremely slowly, so you may not notice the hearing loss. I finally noticed it in myself once when I put my head down on a pillow and suddenly couldn't hear the TV.
There is a good medical solutions to it though--"stapes mobilization surgery". Low risk, better hearing, and tinnitus gone.
So if you have tinnitus and somewhat muffled hearing, see a doctor and mention otosclerosis. It is definitely worth getting checked out. It can really improve your life.