If your preamp outs are tied together in order to provide two sets of jacks instead of just one set, you may create a problem with a Y connector. I believe this can cause the output jacks to the main speaker amplifier to be in mono, so in essence three mono channels. Almarg would certainly know if this can happen.
You stated on 5/27 your system had "great impact" and now with the sub you chose you have lost that? Impact, slam whichever you want to call it IME happens in the 60-90hz range not at sub frequencies.