The Sandisk players seem to work best with headphones or earphones that are between 16 and 32 ohms and at least 96db/mw. Headphones over 32 ohms but less than 64 ohms may be okay if they are more than 100 db/mw, although headphones like the 63 ohm Sony MDR-V6, and 60 Ohm Koss Portapro don’t seem to be at their best when plugged into the Clip+ or Clip Zip directly, and would benefit from a headphone amp(even one like the small and inexpensive Fiio E6) being used inbetween them. Imo though it is generally best to stick with headphones or earphones between 16 and 32 ohms or so(this doesn’t mean that a 33 or 36 ohm headphone is bad, it is just that the greater it is over 32 ohms the more you should be leery of it for use with a Sandisk player without using an external amp, especially if it efficiency (db/mw) is not over 100. ). I suggest avoiding headphones lower than 96 db/mw, especially if they are over 32 ohms(or else you can use these lower efficiency or higher impedance headphones if you use an external headphone amp. Even the $25 Fiio E6 will support most headphones up to around 150 ohms, and many 300 ohm ones as well). if you try to use a headphone that isn’t really suitable without an external amp you might not get enough volume, or you might have to have the player volume set at maximum all the time to get a decent amount of volume, and the sound quality might also suffer.