active or passive?

Listening is usually the correct solution to such a question. Unfortunately, it's not always possible unless you live in an area with many high end dealers. The first limitation is that passive preamps do not work well with sources that have a low output level.

You can use a passive preamp when you have a buffer like an active cross-over IMHO. Otherwise you loose dynamics.

Passive preamp in most cases will not suffice for KSA.

Marakanetz, interesting answer, mine works great with my ksa-250. The ksa-150 is even better match. What did you base your answer on?

low sencitivity 1.7V for full output power requires the line source to have an adequate or higher which is not often the case...