Neil, it'd be possible to arrange the 320 so that the pre-outs could feed a sub amp. The connecting links to the 320 amp would have to be removed and replaced with a Y-adaptor or cable so that both the 320 amp and the sub amp would receive signal voltages. The sub amp would need two line level inputs(the EP125, for example, has one). There really wouldn't be any benefit from going to this trouble, since the 320 would provide no bass management on the full-range signals it sent to the sub amp. A simpler solution would be to connect the speaker level outputs of the 320 in parallel to the speaker level inputs of the sub with speaker wires. The low-pass filter control of the sub would then be used to try to get a good blend with the natural roll-off of the mains.
The 720 has a fixed 100Hz low-pass filter on its combined single sub out. This may or may not blend well with the mains.
