Preamp Output Impedance for my Amp


I have a powermaster 2000 amplifier with the specs located here:

(It appears that input impedance is "25K + 25K"

What output impedance would be ideal for a preamp sending a signal to this amplifier? Also, what is the math required to find the answer?

Thanks for your help!
Any normal preamp should be fine. A ratio of 1:10 is usually said to be the minimum. Most preamps have output impedances in the low 100's, though Ive seen a couple that get up close to 2kohms. Even those should be ok.
"Low" input impedance for a power amp is more like below 10K ohms down to the 600 ohm pro standard. Agree with Paul, you shouldn't worry about this.
thanks, folks. Saw a number of posts to the fact, and started to get worried as I continue the search for a preamp. Y'all have been great help in my search!