Solderboy23Of course it can. I use it connected to the analogue outs of my standard amp at home or (a second, portable unit) connected via 3.5mm jacks to my portable player on the road.
Solderboy23you can but you will be double amping the signal coming out of your phone's 3.5, which is already amped. This will result in dirtier sound no matter what you do, make sure when you do stuff like that your phone is on max volume. I still implore you to buy a dac, even a 30 dollar d03 would be better than double amping out of your phone.
UzuzuI would say to experiment with the volume on your phone. You are going to want to adjust it to something close to line level. Too low and you won't get enough volume to the headphones and too high will introduce distortion. I generally don't run anything at max.