Sorry, but I don't understand your post completely. Are you saying that the ATC 100s have an input impedance of 100 Kohms or are you saying that the preamp that you are considering has an input impedance of 100 Kohms?

I've never seen an active speaker with a 100 Kohm input impedance. The input impedance of the preamp is irrelevant to driving the active speakers. You need to know the worst case output impedance of the preamp. Typically, tubed preamps have rather high output impedance and so the input impedance of the load needs to be high as well.

Recall the general rule of thumb that the input impedance of the load needs to be at least 10X that of the source output impedance.

I've never seen an active speaker with a 100 Kohm input impedance. The input impedance of the preamp is irrelevant to driving the active speakers. You need to know the worst case output impedance of the preamp. Typically, tubed preamps have rather high output impedance and so the input impedance of the load needs to be high as well.

Recall the general rule of thumb that the input impedance of the load needs to be at least 10X that of the source output impedance.