I have to say, I'm not sure you're going the right way with this. Cross-over circuitry is designed to separate a signal into two components - high frequency and low frequency - in such a way that the two outputs added together recreate the original signal. You're effectively using two different cross-over circuits (one in your subwoofer and the other in your main speakers) to generate the two different frequency components. It is unlikely that the bass filtered out by your main speakers will match the bass selected by your subwoofer cross-over. I think you'd be better off using the same
cross-over circuit for both parts of the signal.
What's more, your main speakers are likely to sound better if you don't ask them to produce sound right at the lower limit of their range, and your power amp will be less stressed if it doesn't have to deal with high-amplitude deep bass signals.
For stereo sources you could just use the SP1.7's DACs rather than using bypass mode. But for DVD-A or SACD this is obviously more of a problem.
There are basically three possible solutions:
1) Get a decent DVD-Audio player which does a better job of bass management
. Perhaps an Arcam DV27A?
2) Sell your SP1.7 and buy a Lexicon MC-12. (Possibly a little pricey!
3) Set all speakers to Large and use some sort of external bass management device. There are lots of two-channel possibilities, not as many 5.1 possibilities, but there are some. But you could try, for example, the rather histrionically named "Outlaw ICBM" - click here
for more info.