I am thinking of buying a set of Fluance SX-HTB speakers for home
theater and music listening. Then I want to build an infinite baffle
subwoofer system (1 or 2 Adire Shiva or Tempest drivers) in the wall
of the room, which is concrete block with a doorway that will be
covered with concrete board and drywall.

My DVD/CD player is a Sony DVP NS400D which seems to have some bass
management features (pdf manual at amazon.com). I haven't purchased
the AV receiver yet either but am thinking of a Sony ES STR-DA2000ES,
or maybe an Onkyo TX-NR801 if I can find one cheap in Canada.

Question: Since I will have only 5 speakers plus sub, can I avoid buying a plate
amp and put the spare amp channels of a 6.1 or 7.1 receiver to use
in powering the sub? 

If not, I'll spend less on the receiver.

Comments on any of the above are welcome.

Don't use the extra channels to power the subwoofer cause the
performance based on the RMS output of your amp would be mediocore at
best.  If you're choosing either a Sony or Onkyo Rec, pick the Onkyo
cause it will have a higher RMS output (which means the given amount
of power the amp. is able to produce continuously).  You're better off
getting a powered subwoofer and hook it into the subwoofer output of
the whatever amp. you would buy.  Hope this helps.