I guess ur right, but the Fiio E6 is all i can afford. I'm 16 and don't have a full time job so i don't earn much money. Which cans under $200 would you recommend that would work well with the E6 and have lots of bass (better then average) and still maintain overall balance.
I think as mentioned, I would go ATH-M50. If you lack bass, the E6 should be able to push a little more force into those.
The reason I'm saying that is because you can get the M50 relatively easier compared to the rest of the list, plus if you are willing to get it used, you can find it for around $100.
It turns out that you should be able to get sufficient volume out of a 250 ohm DT 990 with just an E6. See here.
The actual drivers on the DT 7/8/9 series are different for each level of resistance, so get the highest impedance version you can and enjoy it now. You'll get at least equivalent sound and much better comfort now and better scalability once you step up to something like the Fiio E10 or new Audinst HUD Mini.
I wouldn't recommend going over 250 Ohm for anything lower than a Fiio E11 (which can still clip if it's trying to drive a 250 Ohm headphone to higher volumes).
Impedance is nominal at 1KHz, so it may be very different elsewhere, especially at lower frequencies. What that means is that a 250 Ohm headphone may still have an impedance spike of more than 300 Ohm when it's trying to reproduce certain tracks.
I know for a fact that my Fiio E11 can't hold a candle when the DT990 is driven to "movie-worthy" volume. Beyer headphones love... and need a lot of voltage to shine, especially the higher impedance ones.