T1 is semi open, just a little hole in the middle of housing, something around 1cm in diameter ...
and that hole is easily covered with foam or other sound absorbing material , so t1 can be easily transformed into closed back design.
by the way i know what you mean by saying t5p sound strange/unnatural , earlier models tend to have a colored sound , feels like soundstage is in the dark and all musicians / instruments are lighted by a strange spotlight , thats the best way i would describe it ...
many head-fiers noticed that "colored" t5pish thing ...
but later models had less of that and more of treble energy, so people started complaining about t5p sound thin and bass light
then somewhere in middle of 2015 t5p changed a character to a smoother sound with more bass, warmer and darker but it sounded dull to me.
i know all that from personal experience, i had 4 different pairs of t5p with serial number 70*** to 24*** and i noticed some changes in sound character.
right now i am sitting with new 2end gen. drivers and i must say improvements have been made, its still a t5p sound character but with many flows gone.
Its a warm lush sounding headphone, very detailed and revealing, organic sounding but i wouldn't say its neutral. For neutrality i go to studio headphones anyway, t5p is sweet sounding , simply for pleasure and laying back in my chair , no analyzing music , just enjoying it.
But i can see this type of sound character is not for everyone , hardcore audiophiles will find flows