Earlier this year, I began searching for a pair of full sized closed back headphones. I am a student and primarily wanted them to block out all the noise around my apartment, so that I could study. After a great deal of research, I purchased a pair of Sony MDR-7506. Aside from the fact that they are ugly and have an extremely long cord, I have been very satisfied. I have enjoyed them so much, in fact, that I am now considering making further investment in higher quality audio.
A little more about my situation: I listen to a fairly wide range of music - Classical (orchestral, solo violin, and piano usually), Acoustic Guitar, some folksy and blue grassy stuff, some Classic Rock, and sometimes just white noise. I usually listen from a MacBook Air or an iPhone. I listen to music that is not particularly high quality - from 128kbps mp3 to 256 AAC and often 160kbps streaming from Spotify.
So, my question is what upgrade would make the biggest difference in my listening experience? I have considered higher end headphones (PSB M4U1's and HiFiMan HE-400's in particular, which I realize are open backed), a headphone amp/DAC (HiFiMan EF-2A, Aune T1, Schiit Modi and Magni, and the Topping TP30) and investing in higher quality music (like Spotify premium - 320kbps). As far as budget goes...as little as possible, lol. So, please don't just tell me to get all these things. I am mainly interested in the biggest bang for my buck. I would be interested to know the best that I could do with $100-$150, maybe with an eye to investing a little more later.
Thanks for any advice!