Hey vrod, The Dayton 230 looks to be a nice amp. I'm confused as usual. Why not just use the speaker level outputs from your main amp into the high level inputs on the 230? You could simply cobble some speaker wires with two sets of wires; one for the speakers and one for the sub amp high level inputs. Think of bi-wiring schemes. I've used banana plugs and spades with five-way binding posts. That makes it really easy to have two speaker outputs without the hassle of fabricating bi-wire cables. That could be good for you if you are into pricey speaker cables and want to adapt your current cables to make this work without modifications. BTW, I'd love to see and hear your spectral amp. I bet it's nice.
If I recall, the APA 150 only accepted line level inputs, so Ed's magic cables were necessary to attenuate speaker level outputs as to not create disaster.
Of course, I may be completely misunderstanding your question.