To 'amplify' (sorry!) what Gs5556 is saying, almost all home-audio amps will have input impedances above 6K ohms (the same is not true for some pro-audio amps), and most will be above double or triple that figure - and there is no such thing as 'too high' an input impedance (some are well over 100K ohms) - so don't worry about this unless the amplifier has an unusually low input impedance combined with the preamp's output impedance rising dramatically above the nominal 600 ohm rating at the frequency extremes.
