Unlikely that someone here has used both with that particular set of headphones.
I have read lots of positive comments from owners of Caimans and from Fubar IV owners but don't recall anyone who has owned both.
Here is a related thread...
http://community.whathifi.com/forums/t/470786.aspx
Remember to factor in the cost of a Firestone Audio 'Supplier' PSU for best peformance. (It does make a difference.)
TBH it's probably down to cost and looks at this level and you won't find any dealers to audition both together.
I have only experienced the Fubar and Beresford purely as DACs, and only changed to Beresford for reasons of connectivity at the time. (The Fubar DAC that I had was USB only and I needed Optical and coax too.)
I have listened to my TC-7520 (uprated to LN4562NA opamps) with Sennheiser HD-595s and Grado SR60is. Despite sounding great with both, I am a very reluctant headphone user so my opinion is null on this matter.