Not all of the 9505s had the diablo circuit. Ones made prior to 1995 use the 9500/9300 transconductance stage. These also have a simple J-FET input buffer and the DC servo for offset. Though I can not agree with the statement made about the later diablo models being "better sounding", they are a technically improved design.
Hafler 9505 power amps
Do all Hafler 9505 power amps have the "Diablo", and Diamond" circuits? Or where they implemented on later models where it would then be indicated on the amp itself,or in the literature with the appropriate logo? (as they did with the "Trans-Nova" circut.)