Amp vs Preamp which is more important?

Holy moly,

They are both important.  But consider this: If the preamplifier omits anything (no matter how subtle), it is lost and can never be recaptured.  This is only the tip of the iceberg so to speak. Now extend this singular example to all the miniscule details that must happen correctly before the amplifier ever gets the signal.

The amplifier has to be faithful to this small input signal and produce/amplify exactly what it has been given. All of it's attributes must be in keeping with the loudspeakers.  This is synergy and without it, the reproduction just won't work. 

Addendum,

A truly great preamplifier will not be system dependant. Amplifiers must be considered with the loudspeakers that you will be using. Very efficient speakers may match up well with SET amps, etc. Less efficient speakers will require a more powerful amplifier.  Purchasing decisions must be in context with loudspeaker consideration (s).

BOL