I see on Amazon comments for the X1, it is suggested to change to a lead with less resistance. Easiest and cheapest upgrade route.
A decent sound card that has sockets to replace the op-amps has been the most rewarding money spent for me. Preferably a circuit design with (2) I/V sockets and (1) output socket. It is the only way to test and find op-amps to your liking and get the most of your headphones.