whether a particular preamp can drive a specific amp to full power is, in the end, a different question than whether the pre can drive it hard enough for a particular use, and whether inserting the amp will require channel level adjustments u or down. Almost certainly, the amp will have a different gain than the internal amps of a receiver, and will require an appropriate adjustment in channel levels to keep everything in perspective once the amp is added. But since we never really know exactly how the internal gain staging works in any given receiver, it's just a guess whether the adjustment will be positive or negative, seems to me.
And probably for many if not most of us, whether the preamp can drive the amp to full power has no real meaning. Certainly in my room, where max levels reach perhaps -10dB from reference and I use a sub, I don't likely ever use even 50 watts from the amps driving the LCR and surrounds, so whether I can theoretically get full power out of a 100, or 1000 watt amp doesn't matter. I never will use that much power anyway.