Question about using external power amps in 5.1

[kexodusc]

I just brought my old Rotel upstairs and hooked up to my yammie. I made a few quick observations which led to questions I thought I'd ask here.
First, when playing back in stereo, the Rotel (hooked up to the Yamaha RX-V795a's pre-outs) and the Yamaha sound noticeably different. In my probably biased opinion the Rotel sounds better.
Second, when playing a 5.1 track, the difference between the two units isn't quite as noticeable sound wise. The Rotel clearly has more power and handles better at louder volumes, more bass, etc...but at my normal listening level, the differences were subtle.

I'm awaiting a second hand Adcom amp to arrive to take over the Rotel so I can put it back in my main system. This got me thinking though, with all the hype about timbre matching speakers, does the same rule of thumb apply to matching amplifiers? That is, should you stive to have all external amps of the same make/model, components, or is there more flexibility with amplification? After all, amps don't all sound the same. I assume that the audible differences between amps at moderate volumes is less than the audible differences between speakers, which can vary substantially.

Second, I've only tried hooking the amp to the front main channels, is this the best way to go about this, or would I be better off to connect the power amp to the center channel. I ask because I keep hearing that 50-80% of all sound comes through the center. Would I be relieving more of the burden on the receiver's power supply if I connected to the center, or should I just stick with the mains?
Thanks