Just an update.. After listening to my system with the sub connected using the speaker level I/O (amp-sub-mains). Things just didn't sound "right" (if you will). The bass was much better, but the highs seemed edgy, mids not as sweet, presentation seemed less focused. As an experiment, I reconnected the sub in parallel with my mains. Ok, now the mids and highs are back with the added benefit of bass, soundstage and presentation are better than ever.
Regards,
ck