I think most of this boils down to system synergy.I'd be careful about that- if you pair a bright preamp with a dull amp, the result will not be neutral. I recommend to all our customers that whatever they buy, each piece in the system should stand on its own merits rather than hopefully being corrected for its faults somewhere down stream; the latter is a recipe for flushing dollars down the loo.
The one exception to this is that not all amplifiers drive all speakers. This is true regardless of the amplifier, although there are speakers that are driven equally well by many amps. for more on this:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php