I can't recall what I read a while back, but I recall something about the compatibility of SRM's Ant+ head units with other Ant+ transmitters. I'm thinking the SRM head unit may not be compatible with the Stages PM, in spite of the fact that they are both Ant+. SRM may have a distinctive communication Ant+ protocol or handshake. My suggestion is to contact SRM technical support and ask them what Ant+ PMs their head unit is compatible with. But, be prepared for the answer that their Ant+ head units are compatible only with their Ant+ PMs. In which case you may have to go with Garmin's head unit.