I'm not so sure about the differences. I studied both pretty closely (or as closely as the spec sheets allowed) before I bought my RM30s with PBS mounted in the OXO. Take a close look. Both use an AB output stage, the plate amp claims class G but also claims AB outputs, so I'll assume that the class G part is the power supply. The specs for the outboard unit say that it uses a "Patented tracking downconverter power supply increases headroom and efficiency" - sounds like a class G power supply to me. I don't see any info on the outboard's input impedance, so I'll call that a wash.
Nope, looks the same to me.