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The pre-amp is probably the most important part of any amp as it that what gives the amp it's signature. It has no amplify the incoming signal just enough usable current for the power amp to do it's job and drive the speakers.
Why the question about the pre-amp? Is it curiosity, or is there a worry about something? I have a simplistic view on integrated amps. The pre-amp determines how the amp is going to sound and the power amp determines how well, or not, without clipping or distortion your speakers are going to perform in your particular room.
I have a Rega Elicit-R and absolutely love it, puts a smile on my face everytime I put music through it. As for the power amp itself it doesn't colour the signal coming from the pre-amps on my Denon receiver via HT by-pass.
It's quite low, 8 watts (??) or so. I don't believe it's a percentage of the overall power figure of the amp itself. That's not on the pre-amp but the main power drive. Doing a Google search for AB amps yields a lot of results to feed you curiousity.