We simultaneously measure photoresistance with electrical transport and coupled plasmon-cyclotron resonance using microwave reflection spectroscopy in high-mobility GaAs/AlGaAs quantum wells under a perpendicular magnetic field. Multiphoton transitions are revealed as sharp peaks in the resistance and the cyclotron reflection on samples with various carrier densities. Our main finding is that plasmon coupling is relevant in the cyclotron reflection spectrum but has not been observed in the electrical conductivity signal. We discuss possible mechanisms relevant to reflection or dc conductivity signal to explain this discrepancy. We further confirm a trend that more multiphoton features can be observed using higher carrier density samples.