For sure I posted it somewhere on this thread or a similar one, but being unfamiliar with op amps I simply let Sanjay know which headphones I'd be using the BSG amp with primarily. At the time I ended up with one specifically designed for the Beyerdynamic DT 880 600-Ohm version.
As it turns out I ended up using them primarily with my Etymotic ER4P with the S cable, and that's exclusively what I use them with now. It does unbelievable wonders with those headphones. No portable unit has ever made them sound good, but this BSG makes it sound as good as home stereo unit of any quality, costing 10 times more. Really brings the bass out and shows you what that Ety is capable of.
Mine is set up with the 9 Volt, which tends to last at least 10 hours, & I can tell when it's ready to be replaced as the LED begins to dim. I also have a great wall unit for it, which tends to actually power it a little bit better than a new battery does. But it works perfectly awesome with just battery power. I run it with a Nano and a line out jack. I have the internal gain switch jumped for the highest setting, which works better with those IEMs.
Makes a stunning hi-end stereo unit on the go, and I've never been disappointed by it even once. It's also built like a tank. The case looks like it's been through hell and back, and it still works flawlessly.