DE, you may have been making the crossover question unnecessarily complicated and should indeed apply the KISS principle. To me that would suggest that you try the provisions that the SVS has before considering any type of external crossover. Since your 3480's outputs are unfiltered full range, use the sub's internal crossover to set it's rolloff(bearing in mind that, as Tom Nousaine has often found, the markings on the dial may be very inaccurate). Although the details aren't entirely clear(possibly ask Tom at SVS)my understanding is that the line level output on the sub has a 12dB/octave high pass crossover at about 80Hz to the receiver inputs. Feeding this into the amp ins on your 3480(with the jumpers disconnected, of course)might well give entirely satisfactory results without further complication.
