[tacgunner1]

Hey head-fi, I am a headphone newbie searching to upgrade my current set up. I have the Ath-A900 plugged into a HT-R330 onkyo receiver (acting as amp). It's nice and all, but I was hoping to upgrade my headphones and replace the amp with a portable one. I would like the amp to be sub-200$ (it can be used), something that can be used in a stationary set-up (like at a desk with a laptop as a source), or on the go (such as with an mp3). This brings me to my second confusion: I would like to upgrade my headphones with a better pair (sub 300$ and used). I was thinking of going with an IEM, but was wondering if the sound could match that of my headphones. I found a pair of SE-535's for 320$. So my main questions are:
 
1. Would IEM's be an upgrade to my Ath-A900's, or would I need to stick with over-ear headphones for good of quality (at least on par)?
2. For closed ear headphones, what could be recommended (instrumental, pink floyd, alt rock, dubstep, etc. pretty much everything)? 
3. For IEM's, what are some good choices?
4. Would an lossless mp3 with a built in DAC like the Fiio x3 be an option for a portable or stationary set up? Would I still need an amp? Which one?
5. If you would suggest a certain headphone or IEM and optimal amp for it? (best combination), keeping in mind my price boundaries.

 

[tacgunner1]

Would like to mention that I found audio-technica ath-ck100 for 250$, probs can get the price down a bit. My main two are the ath-ck100 or se535, however, if anything is better in this range, please tell me