16 or 8 ohm speaker ?

[Terry]

I'm wondering if it's worth trying to track down a 16 ohm original spec as I'm not sure how tolerant the amp  (AC128/176 output stage) is of the load differential ...

To calculate the peak output power, take the supply voltage and divide by 3 (strictly speaking it should be 2.828 but the supply will almost certainly sag at maximum output) and square the answer. Then divide by the speaker impedance:

P(W) = ((Vs/3)^2)/ZΩ 

To find the current*, divide Vs by 3 again and divide by the speaker impedance:

A = (Vs/3)/ZΩ 

then compare with the answers with the output transistors  data to see if any damage is likely to result.

* This is the RMS value - the peak current will, of course, be Vs/ZΩ