Having brought home a BBflat Bass for the summer break, I'm trying to blow sonorous sounds form the beast. So far, I can play reasonably well from a written middle C up an octave.Playing notes lower than the middle C is proving impossible for me. My lips (chops?) are as slack as they can be and vibrate mightily, but the notes will not go any lower. I'm assuming that the fingering is the same as for other valved instruments. I would appreciate advice on extending the range downwards.