This is going to sound more complicated than it is I'm sure but here goes. I have a 6505 plus combo and two extension cabinets with one speaker in each. All speakers are 16 ohms. If I am just using the amp speaker of course I have the switch on 16 ohms.... when I added the first extension cabinet with another 16 ohm speaker I switched it to 8 ohms. Now I have added a third extension cabinet with a 16 ohm speaker and chaining it from the other extension cabinet because the amp itself only has two outputs for speakers. Question is..... If I have three speakers at sixteen ohms do I switch to the four ohm setting? Or since I'm chaining another speaker from another speaker should I leave it at 8 ohms? Or do all three together make it four? It baffles me and I don't want to screw up my amp! Help me please thanks!