I've been playing trombone for 30 years and was a music major in college before I changed my major. I have a little experience playing tuba as well as other wind instruments. I'm sorry but I'm having a hard time picturing the blockage and tension you're talking about. I've never felt the same tension in my playing as I do when speaking. I'm not saying I don't believe you, just that I've never felt it. You said it's similar to a P sound (I have trouble speaking P sounds as well). Are you feeling it in your embouchure? Or is there tension in your throat? Or both?