On the Kth best base of a matroid

Given a weighted matroid M and a positive integer K, the Kth best base of M problem is to find K distinct minimum (or maximum) bases regarding the weight function. This problem is NP-hard. We prove that it is polynomial for 2-sums of uniform matroids and a fixed number of k-sums of series parallel graphs, M(K"4), W^3, Q"6 and P"6.