Does every piece of gear have it's own unique sonic signature? Logically, unless two components share a majority of the same parts and technologies they should sound different from each other. During 40+ years of listening to audio systems and doing A/B comparisons, occasionally I can't discern any sonic differences between two pieces of gear in a real-time matched-level A/B test. Sometimes, when I've had a chance to live with both components for a while, I've formed a sonic preference for one over the other. What's that about?
The most obvious reason for not being able to hear a sonic difference between two components is that the rest of the system doesn't have sufficient resolving power to allow the differences to pass through it. A gross example of an opaque system is one of those early wireless FM transmitters that had about a 20dB SN ratio due to the background hum. You could connect a Levinson or a Lloyds to it; the sound wouldn't change a bit, and you sure couldn't tell them apart.
Another reason for not hearing a difference is more subtle - the nature of an A/B comparison - switching back and forth doesn't give your ear-brain enough time to discern the subtler aspects of a component's performance. You can only notice the grosser and more blatant differences in an A/B test, while the equally important micro-details don't factor into the evaluation process.
In a perfect world every sound component's sonic performance would so far exceed human beings' perceptual abilities that all components would, indeed, sound identical. But we sure aren't there yet.