I can cite two specific instances where I believe the difference was amps. First in the early 90s at Sound Advice in Jacksonville Florida, room four (high end). Kef Reference 105/3 speakers, Krell amp, preamp and CD player. Krell amp 100 wpc class A behemoth. Played REM Murmurs, Radio Free Europe, serious chills at beginning.
A few weeks later they put together a high end theater system. 105/3 s as L & R, 200 wpc Adcom monoblocks and a Lexicon processor. Played same CD in stereo, no goosebumps. Same room, same speakers.
Second I had Emotiva UPA-1s with my Onkyo TX-NR1008 as front end with Kef XQ40s. Preferred the 1008 internal amps for two channel but HT the UPA-1s were an improvement. In both cases I could not say 100% that the amps were the difference as front end may have been responsible. However, I believe it was amps.
Main Kef: Reference 205/2 & 202/2c, Surrounds: Kef XQ40, Velodyne Optimum 12, Integra DHC 80.3, Ayre K-5xeMP, Oppo BDP-103, Bryston 4Bsst2, Parasound Halo A31. Second B&W: 685 (3), CCM618, Def Tech Powerfield 1500, Onkyo TX-NR1008, DBP 2010, Samsung BD-C7900, Zone 2 Klipsch AW650. Sitting still CCM616, Kef...