This is something that I've been curious about regarding the Beta 22. It seems that hum which is noticeable with very sensitive headphones (such as IEMs) is a common problem even when the PSU is in a different chassis. I wonder why, and what's special about builds that don't have this problem.
Wire routing issues, and capacitive coupling issues with the chassis panels to name two. Also, if you are going to use IEMs you want to have the gain set as low as possible. I personally wouldn't use IEMs for anything other than portable use, in a studio, or live on stage, so it's not something I would ever purposely tailor a home headphone amp to. That's not to say you can't use them, but why would you instead of full sized headphones?
When I used to use IEMs, I only used them with portable amplifiers, whether at home or on the go. Once I got into desktop or home amps though, full-sized was the only way to go.
Edited by IPodPJ - 4/2/11 at 6:07pm