arrayfun(@(K) subs(A, A, [b_1(K), b_2(K), b_3(K); b_4(K), b_5(K), b_6(K)]), 1:length(b_1), 'Uniform', 0)
Note that the substituting vector, [b_1(K), b_2(K), b_3(K); b_4(K), b_5(K), b_6(K)] must be the same shape as the vector of variables, A, for this to work.
An easier way to handle this would be
mat2cell( reshape([b_1, b_2, b_3, b_4, b_5, b_6], , 3, 2), ones(1, length(b_1)), 3, 2 )
since really your A is not participating in the process.