Stereophile's Measurements section is where to find the real world, most useful impedance specifications.
In their review of the Cary SLP-98, John Atkinson states in his measurements:
The important measurement here is the 6800 ohm spec at 20Hz. You can see it's substantially higher than Cary's 440 ohms, or the 800 ohms mentioned by Janhavjar.
This 6800 ohm figure is important because matching this preamp with an amplifier having a low input impedance could cause the sound to be bass shy.
In their review of the Cary SLP-98, John Atkinson states in his measurements:
The output impedance in the midrange and treble was usefully low, at 370 ohms, but this rose at 20Hz to 6.8k ohms, due to the limited size of the output coupling capacitors. This will prematurely and audibly roll off the bass with power amplifiers having an input impedance of much less than 20k ohms.
The important measurement here is the 6800 ohm spec at 20Hz. You can see it's substantially higher than Cary's 440 ohms, or the 800 ohms mentioned by Janhavjar.
This 6800 ohm figure is important because matching this preamp with an amplifier having a low input impedance could cause the sound to be bass shy.