Regarding electrolytics, they can connect between any positive and negative point within the power supply. The negative does not have to connect to the chassis. It depends upon what you are trying to do. Connecting an electrolytic in some places will actually increase hum or may have no effect at all. At any rate, if you look in the schematic, you'll see that one is connected with the negative to the center tap of the transformer, and the other is connected with the negative to the chassis. Think of the more common circuit with the field coil on the positive side. One electrolytic goes before the field and one goes after it. With the field coil in the negative portion of the circuit, everything is the same, only this time you're dealing with the negative side of the electrolytics.

Thomas