The FUN was the succersor to the Compass and an improvement from all accounts. I haven't heard the Compass but I can attest to the FUN being a very good match for the HD650.
The FUN is replaced by the NFB-5 in Audio-GD's current range and on paper looks to be another good match for the HD650. I owned the NFB-10SE (its big brother) and had to let it go because it lacked the same synergy the FUN achieved with the HD650. I'm not sure if the NFB-5 is closer to the NFB-10SE than the FUN and have yet to read of anyone pairing it with an HD650.