Best 12AU7 tube for PS Audio Monos

The reviews of the PS Audio M1200 mono amps have been uniformly positive except for the character of the treble which has been given so so marks by most reviewers.  However, there has been little commentary on whether replacement of the Pavane 12AU7 input tube may ameliorate this deficiency.  PS audio personnel have suggested that the amp can be improved with more expensive 12AU7's but like most manufacturers will not comment on which tubes they are referring to.  I would appreciate input from audiophiles who have actually tried different 12AU7's in this amp.