I wouldn't add one of these unless you are trying to correct/adjust a tonal problem that you have with your headphone listening set up. It would be useful if you could tell us your source(s) and your cans.
If you are using computer files or any other player of digital files you probably have some kind graphic equaliser available to you, this would be better than using the X-Tone IHMO.
IIRC both PinkFloyd and Mrarroyo have championed using an X10-D with the XCan. Although I haven't heard it used that way I do have extensive experience of its capabilities from selling and trying/demonstrating them. IMO they were only usefull if something was very wrong with either the source component (e.g. a nasty bright CDP) or the mixture of audio kit used. Otherwise I would leave both X-Tone & X-10D well alone.