Pre Amp or Power Amp

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 ultravioletaudio.com 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.