I agree with GiR,
The reason to use a DAC on an amp is to bypass letting your sound card do the analog conversion. You want to do that conversion in the amp you buy for two reasons. First, the amp will most likely do a much better job converting the bits of digital data to an analog signal that will drive your cans than the sound card will. Secondly, and probably most importantly, analog signals are very susceptible to RF interference. Digital signals are not. By hooking up your amp via USB to the computer you are allowing the computer to send the digital music as digital data straight to the amp's DAC. The signal then is converted into an analog signal in the amp, far from the RF noise that your computer would otherwise bleed into your sound card's analog signal.
DAPs like an iPod won't speak "USB" so you will use the line out on them to your amp. This is still an improvement because DAPs usually have very clean and clear line out signals. It's usually the amplifiers that are poor. So by taking the line signal and amping it externally, you usually end up with better sound.
Originally I was looking for a better sound from my current portable device, an iPhone. The iPhone headphone port contains a fair amount of hiss. So since I was using Shure 530 IEMs on the iPhone, I did some research and almost ended up buying a RSA Tommahawk. The Tommahawk is reported to not only be a very fine amp with amazing runtime on the battery, but also is said to have nice synergy with the 530s.
After reading a bit more, and realizing that I would like to get an amp with a DAC so that I could enjoy really nice sound off my laptop also (I do listen straight from iTunes a fair amount as well), I researched the RSA Predator as well as the Headamp Pico.
Both amps are very fine performers. I must say that right now there seems to be more Pico love going around on the threads than Predator love, but so much of this is completely subjective that it's hard to tell how much of it isn't just "flavor of the month" effect. Anyhow, I don't really want to take side in the predator vs pico debate, as I've read enough of that and it gets tiring.
I will say that I bought a predator based on two reasons. 1) I was able to get one immediately, there was no wait time. 2) When I asked Ray questions as well as Justin, Ray was much more responsive. I ended up talking to Ray on the phone twice, and he was very pleasant. The one email I shot off to Justin took him 5 days to get to, even though all I asked was what the lead time on a pico was (which he said was about a month at the time).
This is nothing against Justin, I've heard he's a good guy. It's more that I got such great customer service from Ray even
before my amp purchase that it really put my mind at ease about paypal-ing off $450. Read a couple
threads like this one and you'll think twice about pre-ordering from someone that is reluctant to answer your questions (Justin doesn't fall into this category).
Anyhow, after my amp arrived, I decided I wanted some new cans to go with my home setup now that I was using the DAC on my amp with my computer. And after listening to five pairs at guitar center, I choose the DT770-80s. I wouldn't go much higher in ohms than 100 for my RSA Predator. I start to hear clipping on my AKG K240 300 ohm headphones with the portable amp long before it's at an acceptable volume. From what I hear, the battery in the pico is a little higher voltage, so it doesn't clip as early but at a cost. The runtime for the predator is around 55 hours, and the pico is around 20.
Anyhow, I hope this diatribe on my purchasing decisions helps you out. I *still* think you would be better suited to get the 80 ohm versions. You have always been talking about portable amps and players, and to me a 250 ohm set of cans just doesn't mesh well with portable audio. Maybe you should not worry so much about what they look like!
