If you have 515 and you like them, stick with Sennheiser, 600 would big upgrade, but it's above your budget.
Massdrop's AKG 7xx are good phones, but I'd argue they "excellent value". Massdrop doesn't offer returns after item shipped and AKG K702 (made in china, just like K7xx) cost about same on amazon.
Keep in mind I own K702 pro (but I chose to buy Austria made version) - it was almost double the price of version sold now - I love the sound.
One generally agreed great phones are these:
https://www.amazon.com/Beyerdynamic-990-Premium-32-Headphones/dp/B0024NK344/ref=sr_1_3?s=musical-instruments&ie=UTF8&qid=1473539608&sr=1-3&keywords=990+headphones
32 ohm version would be easiest to power from practically everything, while 600 would be hardest.
They are masochists out there that purposely buy 600 version and must use expensive amps (just to get last 2-3% of sound quality)
Lastly - these are really great deal (but you'd likely to need headphone amp to power them - it's ok as they really compete vs $400-500 headphones, not vs similary priced)
https://www.amazon.com/Fostex-Professional-Studio-Headphones-Semi-Open/dp/B0167XM092
Make sure you're only getting the mk3. You'd also need to replace the ear pads to these (it's really easy process)
https://www.amazon.com/Shure-HPAEC1540-Replacement-Alcantara-Headphones/dp/B00HXAV0UY/ref=pd_sim_267_1?ie=UTF8&psc=1&refRID=8JPHN1XRWT57HY44HA58
wait till they are back to $40 (often)