If I follow you correctly, you would in fact, free up one set of pre outs because you would only use one right and one left, leaving the other set open. Running in vertical mode works for some people. My experience was different. When I vertically biamped, my separation completely disappeared and I lost all my imaging. Try horizonal also, and see what you come up with. (That would use both sets of pre outs). As for using two sets of speaker cables versus one set of biwired, either way would work, if you have the chance, try them both. Running in bridged mode should give you more power which (usually) will give you better dynamics and more powerful bass.
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