Amp and preamp impedance

I agree with the previous advice. The response of a filter is dependent on the impedance of the source and the load. I admire your DIY spirit but I think you need to do a little more research into the problem. There are many fine electronics textbooks devoted entirely to filters where you could begin. First order filters are relatively simple, but beyond that it gets rather complicated. Good luck.

For a first order high pass filter you only need a capacitor in series with the input of the amp.

A typical value for the power amp input impedance would be 50K. But as stated above, typical will do you no good since they range from 10K to 100K and the cutoff frequency is directly proportional to this value. You need to get the input impedance for your amp.

fc=1/(2RC times pi)