I think the short answer is that you can't really route external audio / video through your laptop and / or Jriver, particularly not in realtime, and even if you could, what would you output it to particularly for multichannel audio? And even then the result would likely be no better than what our could achieve with most hone theater receivers or pre-pros.
Maybe if the source originated at the laptop (if it had an internal or USB blu Ray or you were playing an audio / video file) AND somehow Jriver could play it AND apply the crossover / dsp functions / filters AND then somehow output that information to an external device that could convert that digital information to analog to pass along to your powered speaker or your future presumably DIY speakers.
Not sure why doing a diy speaker requires this either, unless they are going to be crossover-less and you specifically want hi or low pass filter (or apply eq) outboard.
In the end though there is a relatively inexpensive and effective device that does all that - the usual home theater receiver or preamp / processor. Personally I would also agree that audyssey or one of the proprietary digital room eq programs that some HT receivers use is probably going to trump performance wise anything you could do manually via Jriver - even if you did your own real time analysis of your room and eq curve into Jriver.
