Ideally matching preamp output to a power amp

Are there any best practices when matching pre-amp output gain/impedance to power amp inputs?  I know next to nothing about this topic...

I have a Schitt Ragnarok, integrated headphone/speaker amp, with pre outs. As an amplifier, it has three gain settings:  20 (26dB), 5 (14dB), 1 (0db) via front panel switch (copied from web page).

I recently purchased a Schiit Vidar for evaluation, with another coming for use in mono-block mode.  In SE mode, the Ragnarok into Vidar is able to push my speakers to better aural satisfaction - seems an improvement.  Hoping that two > one.

What surprised me (again, my ignorance) is that the front-panel gain settings also affected the pre-amp outs, and the overall power amp output.  So far, with limited listening, I prefer the high gain of the Ragnarok into the (one) Vidar.

Is there an optimal gain setting that a power amp likes to see?  Can too much gain cause power amp distortion, or some other harm?  The Vidar specs say:  Gain: 22 (27dB); Input Sensitivity: AKA Rated Output (Vrms)/Rated Gain. Or, 28V/22. You do the math; Input Impedance: 22k ohms SE, 44k ohms balanced (copied from web page).

I'm not wedded to the Ragnarok, but since I already own it and occasionally like to use headphones, I'm not in a hurry to shed it unless I can do significantly better.

Thanks -- Todd