Headphone opinion

For $300 to 400, Grado 10 earbuds are a ground-breaking product, as well as the B & W5s over the ears. Both sound remarkable. I like the 32 Ohm people in Portland--very knowledgable in this space.

Yeah, I have not heard the Beyer T5, but it is supposed to
sound like the T1, which I think is my favorite and my go-to phone among Senn 800s/650s, and Grado PS1000s/RS1s, the big Senns and Grados being more expensive.
The T1s have a great, solid lower midrange and rock solid controlled bass, something very hard to do with phones I believe, along with a non-ringing upper midrange, which I think gives them real refinement. Sometimes the upper highs are a bit sizzly, but it's not too bad and that's the only flaw, and you never have everything perfect with phones.
The Beyer Tesla series is far better and sounds nothing like
their older phones, which I use to use.
If I wanted an Iphone/Ipod headphone, the T5 would sure be my choice.

I have not heard every portable phones out there but here my choice of top three, in the order of preference:

1. Denon AH-A1
2. Ultrasone edition 8
3. Bowers & Wilkins P5 (much more controversial choice)

There are lots of information over www.head-fi.org

Beyer's new T5 is designed to be optimized for portable audio. I have not heard them, but have heard the model they are based upon, the T1, which they reportedly share some qualities with. If that's true, they are certainly worth taking a listen to, given your question.

Sorry the above should read Sennheiser HD 25 I-II. This headphone has been a choice of pros for the last 20 years.

My favorite is the Sennheiser HD-I-II. I have not found anything better. Easy to drive. Extremely dynamic (up to 120 dB SPL). Precise sound with tight accurate bass. Great external noise reduction (-17 dB). Extremely comfortable and light weight and will not slip off.

Caveat - this is a pro headphone. You need to like precise sound rather than boom boom tizz.