The input stage of MC275 re-issue is completely different to old design. In balance input of re-issue, the positive and negative phase is push-pull amplified directly and coupled to output transformer to form a completed output waveform. This new design is to avoid the phase-splitter design in the traditional tube amplifier design. In single-end input of re-issue, the input itself forms +ve phase and use a half of 12AX7 to invert input to form -ve phase. In this case, the signal of +ve and -ve phase is not mirror inverted since -ve phase is suffering distortion produced by an extra 12AX7 amplifier stage. This can explain why the balance input sounds better than the RCA input in the case of MC275 re-issue.
Schematic of MC275 re-issue see http://farm5.static.flickr.com/4037/4204605299_bdfa12827f_b.jpg
Schematic of MC275 re-issue see http://farm5.static.flickr.com/4037/4204605299_bdfa12827f_b.jpg