i use(d) different class a amplifiers with headphones. most of the amps i use are nelson pass amps like the the first watt j2 or diy f5, diy pass sony vfet. for headphones like the abyss or lcd4 you don't need a box (voltage divider), only check that the offset is ok and if you like double safty first power the amp on, wait 30 sec then connect the headphones, off - first disconnect the headphone, then power off.
today i use my abyss phi most of the time with a pathos inpol remix (10 watt class a tube hybrid amp) without box - it's the setup i like most of all my gear.
from time to time i use my diy pass sony vfet with a box for my more efficient headphones. it's easy to build a box (voltage divider), take a look here:
https://robrobinette.com/RobinetteBox.htm
but buy some quality resistors (i use some jantzen resistors).
btw 80-90% of the integrated amps using a voltage divider for the headphone socket, well made with good parts it's a good solution and often better then a simply opa headphone ampboard.
imo a good class a amp with around 10-25 watt is hard to beat for power hungry headphones, only headphone amps like the dynahi are similar good.
if your into diy audio a pass f5 is a great amp for these headphones, sadly it's hard to get the parts for a pass sony vfet but thats a real magical amp. a f5 is easy to build and to set the bias, you can made even a better match lowering the power a bit (look at diyaudio com for help).