Many products (Brand "O" for example) support BOTH. XM or Sirius ready only means that there is a connector on the back that will communicate with an external tuner and allow the receiver or preamp to control the tuner and display metadata on the LCD or OSD. The XM and Sirius interface is really just an RS232 connection and although the connectors are different they are electrically similar. The only real difference is in the software protocol in the interface. In theory an XM ready radio could also connect to a Sirius tuner with an adapter cable and a firmware update.