The iBasso D2 is exactly what you want: connected through usb to your laptop (and that is where you will get the best sound) or connected analog to the ipod (using the ipod's dac of course, the D2 then being a simple but very good little amp but am not sure if your Grado does really require an amp on top of ipod's headphone socket).
The D2 works on rechargeable battery, so is completely portable. Recharges when connected to usb....
As a dac/amp for your laptop, this gets you the ability to benefit from great quality, assuming you use lossless or high bit rate files. It is limited to CD input (16 bit / 1411 kbps), so will not do full justice to the 24 bit files you might have. Not sure if this is a D2 limitation or a usb limitation.
Ah, the reviews: not sure about the "star" system here: seems that it is managed in a way where anything below 5 stars should be regarded as "not great", and those 5 stars are flung around all over the place.
I can assure you the D2 is a great little device. Just go for it!