Hi Alan,
  I wish to implement a bi-amplified electronic
crossover for my main
 speakers (the sound card has 7.1 channels so there are enough channels
 for a     5.1 channel system with the bi-amplification) using software
 and my sound card. 
This is definitely possible, see for example
http://rtaylor.sites.tru.ca/2013/06/25/digital-crossovereq-with-open-source…
and
http://www.freedesktop.org/wiki/Software/PulseAudio/Documentation/User/Reci…
Cheers!
Daniel