Last year I did an extensive back-to-back comparison between the Clear, Utopia, Elex, and HD-600. That was before I got the Lyr 3 and RebelAmp.
In brief, yes you need an amp to do the Clear justice. It plays very well on a phone, tablet, or notebook, but gets better with a dedicated amp. I recommend that you stay away from technical amps like the THX AAA 789 and those from SMSL or Topping because they will likely sound too edgy with the sensitive Clear.
I got the RebelAmp about a month ago, and it kicked the staging, details, and dynamics up a notch or two. Per my Utopia comparison, the RebelAmp moved the Clear in the direction of the Utopia. But the Utopia is still in another league. The Lyr 3 has a lot of power and is also good. However, it’s colored by the tube, and not very technical. It’s for warmth, mood, and fun, while the RebelAmp is smooth, full, spacious, and accurate.