You need to make sure that the effective impedance of each branch or group is identical. Otherwise you will get level imbalances as more current will flow through the lower impedance branches. In other words you can forget about the 10 or 5 ohm options. If you need to use both channels, with 4 speakers per channel you can have 4ohms or 16ohms. With a single channel you can get 8ohms, as you have pointed out. The Sherwood will likely be fine with either the 16 or 4 ohm configuration, but there is no way to know for sure without trying it. Personally I would look for a cheap plate amp designed for DIY subwoofers.
