I know we chatted via email, but I just wanted to put an answer here for the larger community. If one person has a question, I'm sure someone else does too!
The XB-300 is powered via the expansion connector. The power supply connects to the DC barrel jack on the bladeRF base board, and you must change a jumper setting (J70) to power both the bladeRF and the expansion board off the external supply (rather than the USB-supplied power).
While it is definitely possible to use the XB-300 with another SDR or hardware, it would take some work; the design was intended for use with the bladeRF.
Generally, we probably won't be able to offer much support with interfacing with other SDRs (as we won't be experts on all the different devices people may want to use) beyond explaining some of the relevant signals and voltage rails on the XB-300. If you are considering taking this effort on, you will need to perform a thorough review of the schematic
before diving in.
The XB-300 has two rails -- a 5V rail and a 1.8V VCCIO_R
rail. This means that if you were going to use the XB-300 alone, you would need to supply both yourself via the expansion connector.
Additionally, there are enable signals for the LNA and the PA, and IO for LEDs. You would need to either pull these to the desired configuration, or interface with them using your custom hardware (being conscious of the IO voltage, of course).