Hi Geoffrey,
See thread entitled "XCSoar Flarm Interface" (I'm unsure how to insert a link to it)! I requested that an altitude filter be added to the program by submitting a "development ticket" on 15 November 2016. It was unfortunately assigned "low priority". You may want to urge XCSoar to implement the request via their forum etc.
I'm now using PAW radar, also waiting on an iOS version of James Rose's RADAR3D which hopefully is imminent.
Regards,
Chris