Here is a prime example. My own system from the past. ARC Reference 40 preamp, 250 amps driving Tannoy Canterbury SE speakers. The 96dB efficiency of the Tannoy speakers did not work to the ARC 250 advantage. They were barely cruising along and not what I would consider The Sweet Spot". This ARC combo sounded much better when it was driving a tougher load like Wilson speakers. The Canterbury SE speaker came alive with the 10w flea powered Shindo SET amp because the Shindo was in the Sweet Spot.
Oh yeah, I got sound but was is it GREAT? NO! I sold that ARC combo to a friend who had Wilson speakers as he would be able to enjoy it much more than I could with my Tannoy at the time. Putting this combo in my other theater room with Wilson speakers was not practical at the time.
https://www.youtube.com/watch?v=7HiS4a-TcAU