Hi.
No I do not use an external amp or dac with the PSB. I own a portable headphone amp SONY PHA-1A, and have tried it with the PSB, but I believe that the amp that comes integrated with the headphone, when switched on, has a better synergy with the PSB sound signature.
As far as the clamping pressure, for me it has disappeared. For a few weeks in the beginning I was stretching the headphones out over a chair that had clothes on it. Doing that plus frequent use has eliminated the problem. The clamping force feels good now and the ear pads are deep with a very comfortable material that feels very good when its not yet warmed up by my ears.
In the beginning that clamping force and the material heating up of the ear pads almost made my ears sweat, but with less clamping force and autumn beginning here in New England i dont have the problem anymore.
I dont have any information about replacement earpads of different material. I have looked online alot and haventbfound anything about sn earpad modification. But two replacement earpads of the same material come included in the box when you purchase the PSB, which makes me believe it is possible to disassemble at home the original earpads.
My head is normal size imo.
Other headphones I've tried besides PSB are Audeze EL-8, NAD VISO HP50, Bowers & Wilkins, Bang & Olufsen, Senn Momentum, ATH - M50x, Sony MDR-1A and a few others that I dont recall the name of.
The PSB had the best sound imo.
As an aside though after purchasing the PSB I discovered Grado and have been using Grado more than anything.
I felt that my entry-level Grado sr-60i were more comfortable than the PSB, almost weightless, and have a more engaging and exciting sound with forward mids and extreme transparency and clarity, with an open-backed design that allow to feel like the music is coming from the room, instead of from two devices strapped tobmy ears the way I feel with the PSB and all other closed-back headphones.
Since then Ive purchased the Grado sr225i and will soon purchase the sr325e, which to me is the best sounding headphone I have heard yet.
At the same time though I still go back to the PSB often for its beautiful tuning and refined bass sound and imo best-in-class clarity and near neutrality. Its an addictive sound signature imo, which I go to often, but not as often as I do the Grado sound.