If you have the extra money I would recommend the Hifiman HE-400 for your headphones. They were already great at $400, but they knocked off $100 since they came out with the HE-400i and now the HE-400 is a steal at $300.
What kind of sound do they have? I listen to a lot of classical, some metal, and vocal heavy music.
Are they a very balances type of headphones? Bass heavy?
Just curious.
That's an interesting mix of genres. The Sennheiser HD600/650 is generally known for a more lush sound that's suitable for classical and less "noisy" music. The HE-400 is geared more toward pop, rap, metal, but EDM is where the HE-400 shines the most.
Relatively speaking, the bass is quite strong and good (visceral, good extension, no bleed and quality impact) but again, this is relative. If you're used to regular headphones/earbuds, you'll actually be underwhelmed by the bass because cheaper audio products will emphasize mid-bass frequencies and give more perceived bass, but this isn't what you want.
The reason why I recommend the HE-400 is because of value: They are comparable to the HD600/HD650, but they have different sound signatures. More importantly, the HE-400 doesn't require an amp to sound good. At the HE-400 tier, this actually isn't very common. You'll find people saying that an amp is "required" for headphones like the HD650 or Mad Dog T50rp, but this isn't the case with the HE-400. A $100 amp is still a nice upgrade for the HE-400, but plugging in the headphones straight to your computer is fine.