For the longest time, studios have used the Sony 7506 headphones for studio mixing. I am guessing this is a very accurate headphone and has a pretty good linear response. But boy, is this headphone bright. Most people don't like its "bright" sound. So avaserfi, this confirms your point that linear response and sounding good are two entirely different concepts.
Nowadays, with all things digital, it's really easy to add good EQ effects to your music. Rip all your music to lossless formats, play around with the EQ in your music player on your computer, pass the digital signal from your computer to a good external DAC and then from the DAC's analog stage to a good headphone amp (or the headphone out on your receiver/preamp), and then to your headphone.
I agree with you avaserfi, it's far better to have control of where the roll off and bumps occur with your own EQ. This is why it's better to have upstream and downstream components with linear response, then you can play around with all those bumps and dips to all your heart's content.
Oh yeah, Ray Samuels amps. I've heard a couple of his amps. To my ears, they were rolled off in the highs, but have pretty sweet midrange. Haven't really heard any tube headphone amps.