Headphones

Many amplifiers, especially combos, have terrible headphone amps that are added without much second thought. They will never compare to a dedicated headphone amplifier. Usually the headphone out is an attenuated (lower) signal taken from the main power amp output. Impedance (resistance) is probably the most important thing you should concern yourself with. There is a slew (pun intended) of different headphone types and just as many impedance ratings. Basically what you want to do is adequately match the head amp's output impedance to the headphones' stated impedance rating. They don't have to match exactly but the closer, the better. Also don't base your decisions too heavily on the published frequency responses. They can be misleading and overstated.

Here's an impedance example:
My Baby Blue II's headphone out is rated at 100 ohms.
My Sennheiser HD650's are rated at 300 ohms while the 280's have a 64 ohm rating. It's a close enough match that they'll both sound fine but the 280's will play slightly louder at the same volume setting due to the lower impedance.

I would recommend you either the Sennheiser HD280 Pro or the Audio-Technica M40fs because they're both closed designs (ie. sound isolation), they have a low impedance so they'll match well with most any amp including portable devices, and they sound really good for the price. When I first tried the 280's I had to double check and make sure I had on the right set! They're both just as good for tracking as they are for listening which is a rarity.

Now that my Baby Blue's on the fritz, I've decided to look for a dedicated headphone amp. You should check out the headphone amp threads (if you haven't already). I posted on one earlier today. Eventually you might want to consider units like the Raven Labs PHA-1 (discontinued but still avail. used and NOS), Cafe Walter or Fodera practice amp. They're geared towards musicians and are built accordingly. Hope this helps!
-Brad