12 Amps? That's dedication! Would be interested in your assessment of your amps.
Whilst I largely concur with your comments, I have found that a better quality amp can make a headphone sound clearer, more detailed, etc. I've had a few different headphone amps, and while most of them sounded similar, a small amount of amps (such as the Dynalo), do sound noticeably better. Noise floor, tonal balance, clarity are all noticeably improved over the majority of amps I've come across.