An mp3 input could be passive, but you're right you'll need an amp to push headphones. Personally I think it next to useless, I'd much prefer an iRig or similar and plug into a smartphone for modelling, fx etc
Right on.. It's only gonna sound like a crappy headphone amp. Ten years ago might have been handy in some circumstance, but these days everyone's got an iPhone and an iRig can be had for $25 on eBay, play along with your iTunes etc its all there in your pocket then.. Ps I go walking with a guitar plugged into my phone all the time
Well I tell you what I really like, is a tone pot and a switch that defeats it, which i consider a lead and a rhythm sound.. (The rhythm sound being the lead sound with filters applied)
But not necessary by any means lotta guys bypass ones that are there and consider that hotrodding..