Hey e.heidenheim, I am a bass trombonist. Am I correct in assuming when you say “the vental range” you mean the low range below the bass clef staff with one or both triggers/valves deployed? If so I believe you have to keep the corners of your embouchure firm, this can be counterintuitive as you will probably feel you need to totally relax everything…I feel that the muscles of my embouchure are working harder when I play a low note with one or two valves to counteract the extra back pressure and extra airflow needed to play these notes, more air is needed as you go lower…I find it helps to imagine the lower muscles at the side of my embouchure pushing towards the centre of the embouchure ..in other words, aperture is a bit bigger but the muscles at the side of the embouchure must still be engaged…and Greg’s rule of not pinching the lips together becomes very important down here because the note will not speak at all otherwise, you must keep your jaw open. It is also very important to use the body’s concert Hall as Greg would say, everything in your body below the embouchure must feel relaxed and open. It is tempting to try to force the notes out when you play through the F valve down low and this is exaggerated playing through two valves but you must resist this urge…don’t fight the resistance with tension anywhere apart from the corners of your embouchure.