Pre Amp or Power Amp

Between the pre amp and the power amp which provides the most improvement in sound quality?
Depends on what you have now.
It's really about the synergy, not about one or the other. FWIW, in my experience, a tube preamp never brought the magic that a tube amp created.
The one that cost you the most.
Depending on what speakers you are using I would match the amp first and then go from there. IMO amp/speaker pairing is very important.
In my experience - pre-amp.

Sorry to break up all the witty replies. :)
I currently have a Modwright 9.0 SE Pre Amp and a pair of Channel Island D-200 Power Amps. with Salk HT3 speakers just courious if I up grade what would be most of and improvement.
Every one who said pre amp couldn't be more wrong.I tried that for years on my big rig.
I finally got a pair of tube amps that could rock my JM 936s they are the Consonance 800s a very very good sounding amp. The difference cannot be described. Ask an old fiend Trelja or vist he has the same thinking. I was using a very highly regarded tube pre yes it was an improvement but using the 800s now it seems like Nirvana on steriods.
BTW the monos blocks use 6CA7 out put 4 per side, still drive the JM's with no sweat hasn't clipped on me yet. They world class and very inexpensive (well built). I saw an article on 6 moons about them.
Tube amp not pre period.
Mechans, Playing rock music how could you tell!
Anyone who knows anything about audio will tell you(all things being equal) that a good preamp makes a bigger difference than vice versa. If you don't think so, try playing those tube amps without one.
Koiman, keep the Modwright. Sell the Channel Islands. Get this ASL Hurricane. It has plenty of power for your 85dB efficient Salk's. Personally, I'm not a fan of digital amps.
They are both important, but the amp determines the speaker decision and vice versa, once you have that right you can experiment with different preamps. But they are both important to getting the system right for you.
Early ASL Hurricanes have relibility issues which are well documented.Due your research if you go down that path.