Soundstage and stereo imaging are very dependent on isolation of the power supplies - if they are sharing a common single PSU there may be rail to rail crosstalk. Try adding a CRC (2200uF / 4.7R / 2200uF) for each buffer to help isolate them. Or get a dual mono PSU. Easier than a CRC, even adding 1000uF caps on each rail right near the buffer boards and a small series 2.2R to 4.7R between the PSU and the rail cap will make a big difference.
Hope that helps.
Hope that helps.