I personally believe that amps can sound different. The differences can be subtle and require the rest of the chain to be adequately transparent and the speakers to be sufficiently revealing. Music is an emotional experience to me and I don't always detect the differences right away. Sometimes it isn't until either I have listened for a while or hit a certain passage that I realize something is different.
Last night I tried to find more info on the "Richard Clark Amplifier Challenge". Besides the often referred to link, I could not find much (besides Forum discussions) except Wikipedia deleted it because it was unverifiable. My experiences plus other credible sources in the industry convinces me not all amps sound the same even within their limits. I know others will disagree, perhaps we can agree to disagree.
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, Zone 2 Klipsch AW650.