I have heard the K550 with the Fiio E11, it adds some warmth to the otherwise neutral K550 without changing the true nature of the headphone. The E17 from the impressions I have read here may also be along the same lines but it definitely isn't going to sound like a different headphone!
If you are still interested in the Fostex T50RP, there is a European mod seller eltocliousus whom you can get in touch with. You can have a look at the Audible bond Fostex T50RP impression thread. Hope that helps, I myself am curious to give it a try!
Edited by bala - 3/14/13 at 3:31am