How will a Cary SLP98 800k ohms - output work with a Pass Aleph 2 - 10kOhms input (unbalanced) I think it will be OK ? The Pass is over x10 the output of the Cary.Usually the issue is the output impedance at 20Hz. In this case, the Cary is about 6.8kohms- and so will be a bit bass shy if the input impedance of the amp is less than about 20Kohms- and ideally the amp impedance should be even higher.
Additionally, to get the most out of a Pass Labs amp which is a balanced design, its best to present a balanced signal at its input. There are tube preamps that can do that and are perfectly comfortable with the input impedance of the Pass Labs amps.