first of all , you need good amp with lots of juice to make t5p sound smooth, dynamic ...
with average gear t5p sound thin , almost no body in mids , sound is thin and treble can sound harsh ...
with an amp like beyerdynamic A1 t5p sound thick smooth and very dynamic , on the dark side and kind of lush ...
Source is important 2 ...
Comparing t5p to hd800 is night and day , t5p is laidback headphone , darker sweeter sounding , works fine with all type of music...
it sounds pleasant 8 times of 10 , but never breath taking , hd800 sounds pleasant 2 times of 10 , but it can sound breath taking, it can give you chills / gusbumps on your skin ...
Now hd800 is even more bright sounding headphone with great treble , one of the best treble out there .... few headphones can match hd800 treble , imaging and detailing is exceptional , compared to t5p , hd800 takes it on the whole new level!
Sound is very realistic , pure high-end, hd800 really shows you the mood of music , brings out the energy of every instrument so effortless and so true to the sound and creates atmosphere of music around your ears.
The problem with hd800 is - its even more picky with amps and dacs, and to the music you choose to play...
if you don't plan to invest 2000-3000€ on amp/dac and good cables , don't bother with hd800...
if hd800 is used in system not on the same level as hd800 it will sound boring , thin , bright , hush , weak bass , annoying fatigue treble ...
I recommend you listen to hd800 in a hifi local store in a high end setup before buying them , this headphone is not for everyone ...