Well, if you're going to get serious, then I think (tongue in cheek) that you need at least one amp per set of cans andIEMs that sound best with those particular listening devices.
So, if I could follow my own rules, I'd have an amp for the Senn HD-600s, one for the PM3s, one for the upgraded Shure SE-425s, and one for the 1More Triple Drivers.
I do have a Dragonfly Red attached to the back of my iPod, but I don't yet know how well it works with any of these cans/IEMs, but the point is portability, not ultimate sound quality.
When my s.e.x. 2.1 is done, that will go with the HD-600s, for example. I've heard that combination many times, and I love it for true musical listening.
Half seriously,
Jim
Jim Rebman -- recovering audiophile
Equitech balanced power; uRendu, USB processor -> Musette DAC -> 5670 tube buffer -> Finale Audio F138 FFX -> Cain and Cain Abbys near-field).
s.e.x. 2.1 under construction. Want list: Stereomour II
All ICs homemade (speaker and power next)