12AX7 use in EVO 100 line

Does anyone know the rationale as to why the Prima Luna 100 series uses both 12ax7 and 12au7  (2 each) tubes in the pre section whereas all of the other series (200, 300, 400) use just the 12au7?  I would have thought it would have been simpler to keep it all the same - is it just a cost savings?