The P500 has 8 ohm and 16 ohm taps on the output transformer. The main cab has a 16 ohm speaker in it, as you mentioned. When you plug into the external speaker jack, the main and external speakers are connected in parallel (16 || 16 = 8 ohms) and automatically switched onto the 8 ohm tap. So the external speaker cab should be 16 ohms. If you plug a dummy plug (a ¼" plug with nothing wired to it) into the external speaker jack, you can connect an 8 ohm cab to the amp. That opens up more options.
Why was it designed that way? It was a design decision, there is no advantage in using a 16 ohm speaker. At the time 16 ohm speakers were available but not many cabs with them were available. If someone wanted a 16 ohm extension cab, Oliver Amps had one available. Ampeg offered this option with the B-18N, maybe the choice of the 16 ohm speaker helped their sales.
In terms of maintenance, it's important to occasionally clean the two shunt contacts (small switches) on the external speaker jack with Deoxit. One shunt is normally open, the other is normally closed. When a plug is inserted, the normally open shunt closes, the normally closed shunt opens. Oxidation on these contacts can affect the performance of the amp. The oxidation acts like a resistor, reducing the output. If it's bad enough, the signal to the speaker can be cut off. Cleaning these contacts will help keep the amp performing to spec. Insert a plug in the jack, this opens one shunt, apply the Deoxit and scrub with an inter-dental brush. Remove the plug and scrub the normally open shunt contact with the brush.
Hope that this helps Carlo.
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