Again, you're going back to IEMs, which is outside the scope of this thread, and I stated I have little interested in using them for anything serious. I'm looking for a *portable DAC/amp combo*, which would directly imply I'm intending to use the setup with a laptop and a larger, harder to drive set of cans. If using a solo amp, it's also amplifying background noise of a laptop's audio out as well, many of which aren't exactly "clean", which is why you'll be hard pressed to find a SNR of the entire audio section of laptops - even desktop motherboards.
I said modest headphones/IEMs. At least read my post.
Anyway, headphone amps don't work that way. They don't boost the signal from the laptop/sound card, they simply supply the output channel with more potential power. DACs might have a gain adjustment to boost the input signal. You likely would want a headphone amp for larger (say 40+ohm) headphones. Some can get away with no amp with 80ohm headphones but that's a matter of the actual headphone's efficiency, background noise, output power of the playback device, and desired volume level. But you don't *need* a dedicated DAC. The SNR is fine with most modern devices (phones, mp3 players, laptops, tablets). Get an amp if what you have simply isn't loud enough. Plus, as I said, any detail gained in using a DAC is negated by the crappy playback environment and other equipment (headphones).
Build a sound room/studio, then maybe you'll be able to discern the difference. Until then, buy what you need based on the actual listening parameters.
Headphone amps can be found for relative cheap. DACs can still be quite expensive because they're intended for higher end (purpose) uses, and are literally sound cards in-and-of-themselves [by design they circumvent the existing audio playback device(s) and/or use a dedicated digital output that circumvents the D/A converter of the existing audio playback device].
