Seems like 100k is a de facto standard for pre-amps pots and attenuators. If most of these have output impedances below 1000 ohms and the favorable ratio is 1:10, why is this?

You could exceed voltage limits if the load impedance were too high, or you could exceed current limits if the load were too small. In many cases, you have a pretty wide working range, and in that case the load impedance can be set as needed, within reason, perhaps anything above a few hundred ohms. But in some cases, the source limits can be reached with values much narrower in range, maybe 20k on the low end hits a current limit and 250k on the high reaches a breakdown voltage. The only way you can know is to study the particulars of the output and input circuits, especially the active devices. Capacitors have a breakdown voltage, so while that isn't usually an issue at preamp levels, it could be in some tube circuits.

There seems to be a bit more gain and the bass is better defined compared to the 100k. I'll leave it in the passive and use the 100k in the Aikido which I have resumed building and hope to finish soon.