Since it is based on Ohms law, the Math says the speaker will last quite a long time as the effective load on the 8ohm output will half. Output should be in the range of 26.8Vac, 16 ohm load would draw a typical range of 1.67A (assuming we treat the impedance as a pure resistance, the 16 ohm G12H75 cream back is actually 12.5 ohms so the current draw on the OT will be in the 2.14A range so the actual resistive load is 57W. Peak RMS current from the Mark V output into an 8ohm speaker can reach 110WRMS. Maximum punch of power delivered to the 75W (12.5 actual) 16 ohm load will be under 71W. Doubt there would be any issue or damage to the speaker......Volume level I would assume would be a bit lower.
No big deal. I have blown 100W -8ohm speakers with the Mark V. Have not tried it with a single 16 ohm speaker.... Perhaps some other time....
