The extra speaker switch could be from the original going bad? Hard to tell without seeing the full shot of the wiring with both selector switches and where they are running to on the amp.
My '58 Continental had the AMP-182, which sounds good, but was lacking in the bass department. I ended up bypassing the "balance" potentiometers (they are bass and treble compensation) on mine. On the bass channel, there is a 100K resistors going to the grid of the 12AT7, for channel 1 and a potentiometer on channel 2. I dropped the 100k down to a 1k, and bypassed the potentiometer on channel 2 with a 1k resistor. The balance was spot on, and I got quite a bit more bass out of it.
Good luck on your build!