Which one defines sound: Pre amp or Power amp?


I'm just wondering which component would influence the sound that comes out of the speakers more?

I have a Rotel 5chx120w 8 ohm power amp, Yamaha rxv1800 receiver and Energy RC-70 speakers for HT. I'm trying to make use of what I have and want to dable in a 2-channel setup.
Here are my options:

a) Buy a Mcintosh mc 252 power amp and use my receiver as the pre-amp.
b) Buy a Mcintosh c2200 pre amp and use my current amp(Rotel).

Which one would yield me a better overall sound?
veltraz81
Upgrade your speakers first.....the heart and soul of a system. Between the pre and amp.....if you can a integrated or older top grade....and have them re-capped. I am a great beleiver in modding older equipment.....reasonable cost and great upside potential.
If your system sounds unnatural, change the preamp. If it sounds weak or thin, change the power amp. To dabble in 2-channel, get a separate preamp. You already have a separate power amp. Take care it is safe to operate your 5-channel in 2-channel mode.
Speakers aside how would the mcintosh c220 pre amp fair with a non mcintosh amp? like a bryston or parasound halo
If i was to get speakers I want a 3-way type. How would these mc's pair with gallo 3.1?