You may not need a stupidly expensive amp for your pre-amp because the speakers you have are extremely efficient, don't need massive power and interact well with your amp, so you can go with a very low power, lesser expensive (not all low power amps are less expensive. I know!!!) amp.
IMO Minorl has made several excellent points in his post, but this one particularly deserves emphasis (although ZD also makes a good point in response, at least when it comes to speakers having especially high efficiency). For a given level of sonic quality, the price of an amplifier will tend to have a significant degree of correlation with its power capability, and with its ability to drive speakers having difficult impedance characteristics. So how a given investment in the amp + preamp should be apportioned between those two components will tend to depend on the efficiency and impedance characteristics of the speakers that are being used.
One important reason why, as I said earlier, that IMO a general purpose answer cannot be defined.
01-26-15: Wlutke
Your amp can't sound any better than what the preamp gives it.
This is true. But it is also true that the preamp can't sound any better than the amp allows it to. So the location of a given component in the chain is in itself of no significance, IMO.
Regards,
-- Al
