Which is more important? Amp or Preamp?

Would upgrading the amplifier or preamp make the biggest sonic difference? Here's my scenario. I currently own a HT setup. Previously, I had what I considered to be a decent 2 channel system (Classe amp and preamp) but wanted to move into HT. After having gone on a recent tweak-fest with cables, power conditioners, power cords, and isolation devices, I only realize now how good two channel sound can be. So what would my next logical upgrade be? I'm considering two options: First, purchase a considerably better 2 channel amplifier for my front speakers for two channel listening (and using the current multichannel amp for surround sound) or option 2...look for a better pre-processor which excels at two channel reproduction. In the end, two channel is more important to me than HT, but I do want the convenience of having HT without having two systems. Here's the list of my current equipment:
Rotel 1098 preprocessor
Rotel 1075 multi channel amp
Arcam cd92 and dv27 cd and dvd players, respectively
PS Audio PS300 power regenerator
PMC OB1 front speakers (along with various B&W surround speakers)

I'm looking on spending around $2500 for my upgrade in the USED audiogon market. I'm leaning towards the 2 channel amp upgrade (YBA, Ayre, Bryston, etc), thinking it would make the biggest difference, but I'm not sure. Any suggestions or recommendations for amplifier or pre-pro upgrades would be appreciated!

Thank you!