100-600 ohms is very high, a lot of great headphones with lower ohms and most common ones 24-32-64, a few 99/100 ohms, so I think they are just trying to excuse themselves if anyone finds the tone lacking via headphones - in other words, blame the customer's headphones not the Line 6 gear - just my guess and too badI don't get this at all, and I use my HD500 exclusively with Sennheiser HD201 headphones. Sounds awesome. No fizz. No boom. They have a nominal impendance of 24 "measly" ohms. Not 150-600.
Some of the best, most even-sounding headphones I've ever had. I think the most important "EQ" to dial in on the HD series is the cab and mic choice. I've found the Greenback 25 to be useful across models and you just adjust the mic to taste. I like the 409, personally.