Sound quality impact - preamp vs power amp?

Preamp gets the nod, as regardless of the amp the sound quality can be no better than what the preamp feeds it.

Yes, a high sound quality amp can sound better with the same preamp, however, that is a result of the the lower sound quality amp NOT achieving the sound quality feed to it from the preamp.  

Cost wise, a match of preamp & amp sound quality will result in the best sound quality for the BUCK. 

Tubed or solid state.  A great tubed pre- is going to be expensive, but you can pair it with a relatively inexpensive ss power amp and get amazing results.  

If you're going ss, then the world is your oyster.  I think you can find good ss pre's for not too much money.  An example would be the Wyred4Sound STP pre new or a used Tom Evans Vibe/Pulse (my current pre which I've got mated to a relatively inexpensive tube amp) And then you can find great ss or tubed amps for not too much $.  

I had an EAR 834 mated to a First Watt J2 and it was one of the better combos I've had.  My best combo was the Vibe/Pulse and a Berning 300B Siegfried.  The biggest regret of my audiophile career was selling my Siegfried to a good friend.  One of the few pieces I've had that actually appreciated in value.  

Meant to say EAR 868.

I have a small 30w A class tube pse amp with matching preamp (same manufacturer) and speakers that do not drop bellow 7.9ohm.
Same 'small' tubes in amp (12ax 7) and in preamp.
The impact on sound is much more felt with choice of tubes (same small ones) in amp than in preamp.
I believe that without any doubt everyone should first choose the matching amp for their speakers (as best as they can find) and than repeat the same with preamp.
Of course, there is plenty of room for mistake(or improvment) in either case.
I do not believe that you can have one 'bad' piece of equipment (cables incuded) and hope that somehow you will 'straight' everything up with another 'good or better' piece, no mattter how that part might be 'good'.
Also, regardles of some belifs that you can use source instead of preamp my experience is that any system will benefit with good preamp (had Burmester 001 and DCS Puccini with clock and both played 'better' with good preamps, Burmester 011 and Arc Ref 3) but that, of course, costs more

On my system, I get more sound difference from power amps than preamps. 
My power amps - Plinius SA102, Audio Research 150M.2, Jungson JA99C. 
My pre amps - Plinius M16-P, PrimaLuna Dialogue Primium, Jungson JA1.
On my most recent upgrade path, I decided to upgrade Plinius SA102. 
Anyway, your system will be always bound to the weakest link. The power amp and the preamp should be comparable.