I am late to this topic
I am comparing the Slew rate of a Hafler Pro5000 to other Hafler amps, and noticed that the slew rate is low
Hafler pro5000 (1991 $1500 ) yes more expensive than the XL-600
SLEW RATE: 1 kHz, 12OV p-p square wave: 40 volts
per microsecond.
Hafler Transnova 9505 (...