Some more things to consider: --proper hammer flange and rep lever friction. Rep spring tension won't be right unless friction is correct. --hammer tail length. On a 'D', 1 1/8" is good. Raising the backcheck is not the same geometry as lengthening the tail. --bore distance. Is the hammer over-striking? Alot? Without custom-bored hammers, the variance in string heights in Steinways can contribute to the problem, especially when combined with short tails and improper friction. I had a bobbling 'D' (1962) here at Dartmouth. The problem disappeared when I installed custom-bored hammers with long tails. Danny Dover Dartmouth College Hanover, NH
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