The op is exactly on target in his 'enlightening discovery'. It's not his imagination. What would further 'knock his socks off', after finding the 'sweet spot', is to match the left channel to the right while listening:)
Is it real or just my imagination?
I recently checked and re-biased my ARC VT100 Mk II power amp. Tubes had not been re-biased since new, and the hours had reached around 500, so I felt it was time to check and adjust if necessary. Spec is 130. One channel measured 128, and the other 125. I set them both to spec, closed the amped, and took a listen. I was surprised to find that the amp sounded more dynamic, more "alive". I was surprised that this small a change in bias would make a clearly audible difference. Any body else have a similar experience ?