Hi,i'm not sure i see what you mean(if you ever mean something anyway)do you MEAN around the grille of the driver(see pic 1)?If so it already has thin felt glued around it,so i don't see the benefit of surgery tape.If you mean inside the cup behind the driver it has foam all over(pic 2),i don't see where you would stick your tape,and moreover the effect(if any)of it.i think your tape has more to with the famous placebo effect or auto-suggestion.
PS i'm one of those unsatisfied customers who've been fooled by the buzz you can read about the Nighthawks(especially on these pages),the famous burn in issue supposed to reveal the true magical nature of those headphones and all the s**t blah blah around it,but please if you have any REAL suggestion as how to smooth out the highs(contrarily to what 99,5% of what the hi-fi press says the problem of those cans are the little too obstrusive highs for me) i'll buy them,i would personnally suggest placing a thin felt on the middle of the grille where the highs are emitted,
qui m'aime me suive as we say here(France)..
2nd PS i have no permission to insert images(too bad,they were more speaking than any word),but trust me the only place you can place tape is on the grille