It's easy to compare the sound of two amplifiers and choose which sounds closer to the real thing.
It's a lot more difficult to do that with only one.
The reason is simple. In the first case, you're comparing two real-time events. In the second, it's a real-time event vs. your memory.
We all want to believe our memories are pretty good, but in fact, they're rather flexible—adaptable to the moment.
This is one good reason to always have a known reference to compare against. Even if it's just a treasured recording, a trusted pair of speakers.