I own the fiioe6, but understand that I will need more. I have a $75 limit for the amp and was wondering which, if any, would allow me to use these cans at at least almost full potential with that price cap.
Any input is appreciated! Still learning!
The FiiO E11 will do the job decently.
Bravo amps are pretty good. I owned the Bravo Audio Ocean as my first amp and it powered my HD650 pretty well. I wasn't sure if you wanted a portable amp or desktop amp though, since you mentioned running the Beyers straight from an iPhone, so sorry if my suggestion wasn't what you were looking for. I actually ditched the Bravo for the Schiit Vali though. It can power just about anything out there and doesn't cost an arm and a leg. Too bad it doesn't fit your budget. They run about $90 used and $120 brand new. I think you'll be happy with the little Bravo though.
I can't comment on the Bravo other than that tube amps can be hard to design for at low prices... hybrids are a bit different though.
The 250 ohm DT-990 is high ohm, but a nice 96dB/mW sensitivity so it's not too crazy to drive. In fact, they can be easier to drive than the 32 ohm model IIRC - https://docs.google.com/spreadsheet/lv?key=0AhF_x0eVgXlGdDZwdnVCTXRvOV9uQ0NnTHNHOFNBcnc&type=view&gid=0&f=true&sortcolid=0&sortasc=true&rowsperpage=500. I'd try your E6 first before buying something new.