I like the flexible grunt tubes either ones by flextone or at least have a flexible tube extension but the "softness" of the entire call including the mouth piece just sounds more realistic to me. I've had luck with all kinds of calls but again, just like calling turkeys, you need to put some personality into the calling. If you're just puffing air through the grunt... ba, ba, ba with no inflection or tone or variation, it sounds contrived. If you're making a contact grunt, make it, blurp, blurp, blurp, make it sound like there's some wetness or softness to the grunt, not just a dry ba, ba, ba,. I believe you can achieve this with short, staccato type grunts in flexible calls and by moving the call trying to visualize a deer's trachea and moving while walking, running, trotting like they so often do during the rut. If you're just putting the call up to your mouth expecting it to do everything you're not calling right in my opinion.