In my experience, the iHome iBN24 bluetooth stereo boombox is the same across all my devices, and gets insanely loud. I'd recommend the device for you if the Jambra is giving you the same volume limitations as audio cable-based speakers. I'm not experiencing this issue with either my iHome iBN24 bt portable speaker, or my iLIVE IHB23B bt speaker system.
I agree that it's not a solution to require a separate device... But I don't think there's any way to fix the volume issue of the X2's speakers, as I truly believe they're just that cheap/weak. As for the headphone cable issue... This is a problem I experience with Nintendo and Sony's portable gaming hardware, such as the DS/3DS and PSP/Vita. When using headphones on those devices, they are quieter than most MP3 players and laptop/desktop PCs.
Why a volume cap for headphones exists I don't know... But if we want to listen to our devices at stupid-high volumes, we should be allowed to.
