Advis for a pre with 600 ohms output ?

600 ohms is par for the course with tube equipment. As stated above, no power amp should be a problem, but you are more susceptable to problems due to long or low quality (high capacitance) coax interconnects than is the case with solid state preamps at 10-50 ohm output impedance.

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.

A 600 ohm preamp can drive practically any amp on the planet. Buy what you like based on what you can afford. The input impedance of a commercial power amp is a non-factor - no matter what the value - as far as "matching" is concerned.