@mofojo
"Do you prefer the Focals or the Sennheisers? "
To me, one of the most interesting experiences I have had. I got serious about headphones about 18 years ago. I slowly upgraded from basic, with basic electronics. I have LCD2, Beyerdynamic T2, Ultrasond Edition8... etc. I have had many good amps searching for the magic. The differences were typically what you read... Sennheiser trebly not quit of good a bass but airy, the Focals... I forgot, whatever the reviews say. My last set of upgrades from a Woo WA6 se amp, the Aurender streamer... suddenly the differences virtually disappeared. They both have perfect sonic balance, no treblyness, great articulated bass. The only difference is the Sennheiser have open backs so they have a bigger sense of space... no closed in feeling.
My belief is that, both these (and other top headphones) are designed using absolutely outstanding neutral signal sources. But when purchase and reviewes are done on systems with some shortcoming that brings out a weakness or propensity if not fed a really good signal. Hence they get a reputation for a particular sound. I think the Sennheiser need massive current to drive them, and the Focal require real midrange bloom. Anyway, my sources eliminated any weaknesses in both and sound almost identical. The LCDs also sound nearly identical. Maybe, I just stumbled on a particularly synergistic set of electronics for the headphones and my rational isn't correct. But lucky me anyway.
So, the short answer is now the Sennheisers get almost exclusive use because they are more comfortable and more "space" because of their open design.
