The 250 ohm and 600 ohm beyers are the hardest. Because everyone has them...and I kid you not, everyone has said differnt things about them
I have had many say the 250 ohm beyers are driven extremely well with the E17 and E11, and one or two(one recently) say the 250 ohms aren't driven well with it. I have tme over here saying the 600 ohm works well with the E11/E17 and two that say otherwise.
At this point, I basically set a cut off mark. Most people don't use the e17 with the 600 ohm beyers and thus aren't as many input on if it works or not. Many with 250 ohm are using it...and so I leave the cut off at 250 ohm. No sense in playing a 50/50 game with newbies that come.