Recently bought my father an X-Cans V2 headphone amp on Ebay (I know, I know). I've heard that it's possible to improve the sound output of an X-Cans appreciably by replacing the valves with some better quality ones. Would anyone like to suggest any specific valve that would be appropriate for this? Where might one acquire them, and how much money are we talking about? Amd how complex is the replacement procedure?