Will it work?

My preamp has 500 ohms utput impendance and i would like to use a poweramp with 20.000 ohms input impendance. My XLR cables are about 18 feet. Is there any risk that the lows or high will suffer from roll off?
Any input is welcome.
XLR cables are used with studio equipment (600 Ohms input/output rating). Should be fine.