How big of a difference do amps make?

Case in point, in 2002 I purchased both a Krell KCT preamp and 300cx amp from San Francisco Stereo to power my Thiel CS6s. After half a day of listening I decided that perhaps the more powerful 400cx might be a better match, so I swapped out the 300cx for the 400cx and immediately, I heard more detail, deeper and more highly controlled bass, and deeper soundstage (yes, the 700cx was way out of my budget then). My point here is that even within model lines, and given all else equivalent from a design and execution standpoint, the power supply itself and the current served up to the speakers can make a huge, audible difference.