Am using the BSG Cmoy as I write, under the following configuration:
Etymotic ER-4P > S Cable (75 Ohm impedance resistor for a total of 100 Ohms) > BSG Cmoy (powered by DC, not battery) > SendStation Line Out > iPod Nano 4th Gen.
The amp is stupendously tremendously fantastically awesome.
Nano Alone:
Sound is decent, as the Nano uses a "decent" op amp chip, I am told by a Head-Fier who works at Apple, and of course the Etymotics are no slouch. However, the sound can be a little dead in comparison, of course, to running my Nano through a real amp. (I use the Glow Audio One tube amp). The Etymotics are awesome, but they only do what the source gives them, and in this case straight from the Nano they are a little 2-dimensional: great sound, nice panning, but little depth.
Nano w/ BSG Cmoy:
Sound is totally transformed. I don't use the words carelessly: it sounds like a different setup entirely. Clarity is totally restored and musical detail springs to life; in comparison, the Nano alone sounds totally muddled. Increased treble response. Bass deepens in the likes that I've never heard my ER-4 actually do, and I've owned them 6 years. Best of all, the sound becomes utterly 3D, with incredible lifelike depth and extremely broad soundstage.
I never cease to be impressed with what the Etymotic ER-4 is truly capable of when given the right source material, but the BSG Cmoy stretches it to new limits I've never imagined.
Warning:
I ran the BSG with my AKG K701, both stock and with a 235 resistance cable (290 total), and a friend's Beyerdynamic DT880 600-ohm version, and did not much like the sound. I'm a tube guy, so that may be it. But, with the ER-4 IEM as I said, it was heaven. Especially for travel, as I'm not lugging my 15-pound partly glass Glow with me on trips!