amtrpt

Hi,
It's an interesting question and honestly not something that I have thought about a whole lot. First of all be sure that you aren't confusing articulation with tounge position. I try to keep my articulation the same through out the registers. However the syllables that I think of when playing do change depending on the register I'm playing in. For instance if I am playing the upper register I try to think of a more eeh syllable. This helps me to keep my tounge a bit higher and gets the air moving faster. When I'm playing in the low register I want a much more open syllable like aah to keep the tounge relaxed and down so that the air can move at a more relaxed pace. Just be sure not to confuse the two ideas. For me they are quite separate.
I hope that is helpful. Please let me know if you have any questions.
Best,
Andrew