An approximated spectrum (twenty of the smallest and largest eigenvalues) of an example system matrix derived for the polarisation-sensitive case and preconditioned with different preconditioners. From the bottom to the top, we show the eigenvalues of A, M_BDA, and M_{2lvl, { S,D }}A, where M_{2lvl, { S,D }} denotes the two-level preconditioners proposed in Sect. 3.2. The respective condition numbers, κ, are given in the legend.