The AKG k550’s are a fantastic closed back headphone with an open back sound. You can still buy them, but I believe they are now an MK11 version. Also, the Graham Slee Novo is a fantastic little headphone amp. It can be daisy chained into your system without occupying a line input, for instance, placed between a cd player and your amp. It has both an input and output RCA’s on the back that are internally linked to enable this. You might be pushing $600 for this combo. You can read up on both of these products at WhatHiFi.
https://www.whathifi.com/us/akg/k550/review
https://www.whathifi.com/us/graham-slee/novo/review
https://www.whathifi.com/us/akg/k550/review
https://www.whathifi.com/us/graham-slee/novo/review