Try opening your bottom lip more, or have it bunched up and in front of your teeth. It is best NOT to have your bottom lip over your teeth if you can help it. Sounds like the tightness is because you are using your jaw instead of your lip muscles to form your embouchure. You may need to use your lightest reeds.
Just off walking the dog on the Gower for a few hours...............
Yes - I adopted this technique shortly after starting up - you tire more easily but build up strength and control over time. I also went to see Gilad Atzmon in Exeter a while ago and asked him about subtone during the break - typically (nice guy that he is) he gave me a short lesson - start by just blowing down the horn with no intention of getting a note, minimal effort, so that all you get is the breath sound, and then just increase the lip pressure and/or breath until the note forms, but no more. (I am sure long note practice would be good with this, but I get bored with it and played choons instead...) Over time the note will form with less and less effort, and the result will be a softer tone. Another way of visualising it is to try to play as quietly as possible - and your wife/neigbours would appreciate it. I now find it hard to play without subtone if the octave key is off - but that's just fine by me. I would like to be able to play higher and higher with subtone - not the Coltrane skronk for me! Iain Ballamy is another good example of playing high notes with subtone and perfect control.
Google "myspace iain ballamy autumn leaves" for an example. Can't believe it has only had 3 plays.