Which one defines sound: Pre amp or Power amp?

Since nobody has responded to your question yet, I might as well bite.

Before I give my opinion, I would like to highlight that both your options of adding the McIntosh to your existing setup are not too feasible as your other stuff(especially the Energy speakers) are mainly for HT. If you are considering the Mcintosh pieces, you might as well revamp your whole system by getting both preamp and power amp and changing the speakers as well. Then only the full potential of the 2-channel setup can be realized.

If you really insist, I would go with the preamp first, that if your speakers are capable of revealing the finer nuances and detail from the upgrade.

Leaving the Rotel in play greatly diminishes any significant upgrade in the system's performance.

I agree with Ryder. I would upgrade the speakers first, then preamp. I have McIntosh C42 preamp and MC252 amp. Without speakers that can extract the full potential of the McIntosh pieces, I wouldn't bother upgrading to better pieces like McIntosh, Classe, Pass, C-J, etc.