Dev, yes I know. . . but I can only trust my ears, regardless of what the manual says. And in my Ref 3 the SED 6550C in the power supply declines even faster: 900 hrs may be the extent before the sound becomes dull and uninvolving with significant loss of dynamics and treble information. Please note that I am always specifying 'my' Ref 3, rather than to the Ref 3 model in general. . . it is entirely possible that the relatively rapid tube decline that I have observed is a phenomenon peculiar to my own unit.