Is it possible a 20 amp outlet can make a big difference in SQ?

This is a somewhat rhetorical question, but I'm interested if anyone has a similar experience, or can explain why...

I have one duplex outlet to power everything. I assumed it was 15 amp so a few months ago I replaced the outlet with a hospital grade 15 amp outlet. (fwiw there are 4 wires, 2 white 2 black). My tube amp is plugged directly into the wall.

On Friday the air-con went out so I had a reason to check the fuse box - and realized that the circuit powering my AV is 20 amp.

So I replaced the outlet with the same brand and grade but 20 amp. 

I was surprised that there was a significant change in SQ - I'd say dynamics are faster, more detailed, more powerful. 

If I'd spent $500 or more I'd factor in the mental bias but I spent $20 and expected nothing.

Am I imagining this?

Any thoughts?