You should refer to the spec sheets. However, gain can be calculated but only with some assumptions.
The M300: if the 300W are into 8-ohms, and assuming the output voltage stays put, the output voltage is SQRT(300*8) or 49 Volts. If 0.9 volts is required to obtain 49 volts, then the gain is 20*log(49/0.9) = 34.7 db. Seems a bit high, but it's based on assupmtions.
Similarly the gain for the C150: 20*log[(sqrt(140*8)/0.6] = 35 db.
The M300: if the 300W are into 8-ohms, and assuming the output voltage stays put, the output voltage is SQRT(300*8) or 49 Volts. If 0.9 volts is required to obtain 49 volts, then the gain is 20*log(49/0.9) = 34.7 db. Seems a bit high, but it's based on assupmtions.
Similarly the gain for the C150: 20*log[(sqrt(140*8)/0.6] = 35 db.