With a pro studio amp like the SPL Phonitor the HD-800 treble peak is very audible especially with electronic music. Now, to mix or master with 800s it can be very hard. First your mix/mastering can get dark, opposite to the treble peak. Second there is no studio monitor that I know to be boost by + 5 db in 20Hz area. As well Sennheiser never claim for the HD-800 to be a studio monitor headphones.
Is possible to roll off the treble peak of the HD800 with some tube amp, but then the 800s will be not so balanced headphones.