No you can’t. You use either the 2,4 or 8 ohm tap, but not more than 1 at a time. You will harm the amp if you do otherwise.
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Although you’ve found a solution, I’ll mention that I took a look at the schematic for the amp, which is contained in the service manual which can be viewed at hifiengine.com if one is registered there, and I see no reason why you couldn’t use a tap for the sub that is different than the tap the main speakers are connected to. The worst that could happen would be a hum problem, but I doubt even that would occur.
In saying that, though, I’m referring to the red and yellow wires from the sub. Without knowing the details of the internal design of the sub I would recommend against connecting the black (ground wire) from the sub to anything other than one of the amp’s two "Com" terminals. (It doesn’t matter which Com terminal is used, as the Com terminals for the two channels are connected together internally).
Also, if you are using just one sub the red wire should be connected to the same tap on one channel that is used to connect the yellow wire to the other channel. Otherwise sonics would be compromised, although no damage would result. And if you are using two subs the red and yellow wires should be connected together, to a single tap on the amp channel corresponding to the particular sub.