I think Bandit has a very good point that speakers and room acoustics have a larger effect then the upgrade of a amp or receiver. Leave that aside though, as the question relates to equipment. Before I moved to the Bryston equipment, I had Denon, and before that a Yamaha. When I was shopping for the Yamaha I A/B'd Yamaha, Sony, Pioneer and a few other brands at a local home theater shop, all using the same speakers and source. I wont go into which one sounded the best, but I will say that I did hear a difference in all their sounds. Each receiver sounded different, not necessary better, but I could tell something changed. Point is that if it sounded different, then logically just because it's being used within the specs it doesn't mean it sounds the same. There are many schools of thought on this, and many would disagree, they would say that all amps\receivers sound the same. Go to a Tweeter and listen to 4 different receivers, if you can hear a difference,(at the same SPL) that will answer the question if equipment sounds different. I perceived a better sound with my Bryston at all levels compared to my denon 3805. I had also tried a Rotel 1077 digital amp, I really liked the sound of that too, but the upper end sounded "different" then the Bryston, and I preferred the latter. Go take a listen and try before you buy, that's half the fun in this hobby!
Brian