These actually sound rather nice straight from a iPod or iPhone. If you was going to get an amp then yes the Pico would be a lot better than the Fiio but also more expensive. The Pico i have not tried with the momentum but i can see a good synergy between the two as Pico is rather neutral, extremely transparent and slightly bright which will overall help the momemtnum and not hind its laid back treble!
The Pico amp is warm sounding. Pico slim is the one that is neutral and slightly bright sounding.