Advis for a pre with 600 ohms output ?

Following the 10x rule I should get a minimum of 6Kohm power amp, correct?
Which will be then the highest suggested value ratio: 50X ??
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.
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.
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.