How much does power amplifier really matter comparing to other hi-fi components?

the secret is that speakers are the most important element ... BUT YOU SHOULD SPEND MOST MONEY ON THE SOURCE ...

50% speakers
25% Amp/preamp
25% sources (me: 2 TT's stereo, mono, tuner)

This is a massively complex question. It sounds like you want to build a high end system from scratch which can be a big job, particularly with the reduced number of dealers. I'd find an audio dealer whose taste runs to your own and begin the work. What kind of music do you prefer, what's your budget, size of the listening room, do you have any pieces already to carry you for a while.... are all things the dealer can use to guide you.

To  your specific question, in terms of amp vs. pre-amp I'd put more into the amp. As others above have noted, the amp is tremendously important. Say 2/3rds to amp and 1/3rd to pre. Similarly, I'd spend more on the DAC than the transport/streamer.
I agree also with pani that the speakers and amp should be purchased and matched together. 
Good luck!!

Just to be sure, I asked around. Blackhawk said it is the most important, Moab said it is, F1 of course, Sovereign, Koetsu, right on down the line. Can you believe, every single component insists it is the most important! "Listen," I said, "you can't all be most important!" OH YES WE CAN they shouted back.  

I'd believe them if I were you. The Blackhawk in particular was getting pretty heated over it, and the power cords, one went Nova the other went freaking Supernova. I am not kidding.   

i read back many years ago in a stereo review magazine that you should buy the most watts that you can afford!