OK, I've done a lot more focused listening and here are my conclusions, based on my listening with a Sony NWZ-A818 and Etymotic ER-4Ps (with 4s cable):
- Volume with the 4s cable + FiiO amp is almost identical to unamped 4p cable, but sound quality is much better
- With the 4s cable, the amp adds volume equivalent to about 8 clicks on the A818 volume meter -- in other words, level 22 with the amp is about equivalent in volume to level 30 without the amp
Overall, even without the volume boost it gives me for high-impedence phones, I prefer the sound with the amp. Given the price, size and sound quality I'd say this is a no-brainer. If you need a volume control or power switch (power is cut by unplugging headphone jack) look elsewhere. But if you want a tiny, great-sounding amp at an amazing price, I'd highly recommend giving the FiiO a try.
- Comparing equivalent volume levels with and without the amp, I prefer the sound with the amp. Hard to pinpoint what exactly I like better, but it's subjectively better. Soundstage is a bit wider, overall sound seems a bit more precise and open. Bass is boosted a bit, but it's not overwhelming and bass is still tight.
One other side benefit for me: the headphone jack on the Sony is at the bottom, and I'd always kind of wished it were at the top. With the FiiO configured the way I have it, the headphone jack is now at the top.
P.S. I wish there were an apparent easy & reversible way of opening the case to see what's inside, but there doesn't appear to be. The size appears almost identical to Xin SuperMicro and I am wondering if they cloned that amp (without the Xin's volume control).